Highly efficient 17.6% tin-lead mixed perovskite solar cells realized

demonstrated by introducing an n-type interface with spike structure between the absorber and electron transport layer inspired by highly efficient CI...
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Cite This: Nano Lett. XXXX, XXX, XXX−XXX

Highly Efficient 17.6% Tin−Lead Mixed Perovskite Solar Cells Realized through Spike Structure Gaurav Kapil,*,† Teresa S. Ripolles,‡ Kengo Hamada,‡ Yuhei Ogomi,‡ Takeru Bessho,† Takumi Kinoshita,† Jakapan Chantana,⊥ Kenji Yoshino,§ Qing Shen,∥ Taro Toyoda,∥ Takashi Minemoto,⊥ Takurou N. Murakami,# Hiroshi Segawa,† and Shuzi Hayase*,‡ †

Research Center for Advanced Science and Technology, The University of Tokyo, 4-6-1, Komaba, Meguro-ku, Tokyo 153-8904, Japan ‡ Graduate School of Life Science and Systems Engineering, Kyushu Institute of Technology, 2-4 Hibikino, Wakamatsu-ku, Kitakyushu 808-0196 Japan § Faculty of Engineering, University of Miyazaki, Gakuen-kibanadai-nishi-1-1, Miyazaki, 889-2192, Japan ∥ Graduate school of Informatics and Engineering, University of Electro-Communication, 1-5-1 Chofugaoka, Chofu, Tokyo, 182-8585, Japan ⊥ Department of Electrical and Electronic Engineering, Ritsumeikan University, 1-1-1 Nojihigashi, Kusatsu, Shiga 525-8577, Japan # The National Institute of Advanced Industrial Science and Technology, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan S Supporting Information *

ABSTRACT: Frequently observed high Voc loss in tin−lead mixed perovskite solar cells is considered to be one of the serious bottlenecks in spite of the high attainable Jsc due to wide wavelength photon harvesting. An amicable solution to minimize the Voc loss up to 0.50 V has been demonstrated by introducing an n-type interface with spike structure between the absorber and electron transport layer inspired by highly efficient Cu(In,Ga)Se 2 solar cells. Introduction of a conduction band offset of ∼0.15 eV with a thin phenyl-C61-butyric acid methyl ester layer (∼25 nm) on the top of perovskite absorber resulted into improved Voc of 0.75 V leading to best power conversion efficiency of 17.6%. This enhancement is attributed to the facile charge flow at the interface owing to the reduction of interfacial traps and carrier recombination with spike structure as evidenced by time-resolved photoluminescence, nanosecond transient absorption, and electrochemical impedance spectroscopy measurements. KEYWORDS: Tin−lead mixed perovskite solar cells, spike structure, voltage loss, high photocurrent, multication

T

nm.15−19 As a result, the broad absorption range of Sn−Pb mixed perovskite solar cells (PSCs) possesses high-photocurrent characteristics when compared to pure Pb counterparts. As elucidated before, a 50−50 mol % of Sn−Pb has been reported to exhibit higher absorption coefficient, especially in the infrared region more than 900 nm.16 Despite the high photocurrent generating material, Sn−Pb absorber, however, in turn exhibits less open-circuit voltage (Voc),15,19 and according to the PSC structure, this Voc loss can be divided into three parts: first within perovskite absorber layer, second at the perovskite and electron transport (ET) layer interface, and third at the perovskite and hole transport (HT) layer interface. It is important to notice that Voc loss

he promise shown by organometallic trihalide perovskite (ABX3) at the beginning after its invention in 20091 as an efficient photovoltaic material has been made with attaining a record efficiency of 22.7% in 20172 so far, approaching near to silicon solar cells. This tremendous growth can be accredited to the choice of varying the composition of the material3−6 with ease while maintaining its perovskite structure. Of the various compositions, methylammonium (MA) lead iodide (MAPbI3) with a light absorption up to 800 nm has been used extensively7−9 as an absorber in solar cells. The absorption range (800 nm) can be increased in infrared region with the replacement of MA cation by formamidinium (FA) cation to 840 nm.10 Another approach to increase the absorption is by introducing bivalent tin cation (Sn2+) at B position, increasing the solar cell photon harvesting more than 900 nm.11−14 Interestingly, by sharing the B position between Pb2+ and Sn2+ the absorption can be taken toward the infrared region to 1060 © XXXX American Chemical Society

Received: February 18, 2018 Revised: April 14, 2018 Published: April 27, 2018 A

DOI: 10.1021/acs.nanolett.8b00701 Nano Lett. XXXX, XXX, XXX−XXX

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Figure 1. (a) Schematic showing the fabrication of PSC. (b) Complete device with the thickness of different layers. (c) Energy diagram showing the energy level of different layers in PSC fabricated, where PCBM layer exhibits a CBO of 0.15 eV on the surface of FAMA absorber layer.

design as well; an ET layer with 0.0−0.3 eV higher conduction band position (CBP) relative to perovskite absorber layer would be more efficient than ET layer with lower CBP. Device Fabrication. Figure 1a shows the steps involved in the fabrication of p−i−n solar cell on indium tin oxide (ITO) glass (5 Ω sq−1, Geomatec, Japan). ITO glass was given an ultrasonic bath treatment for 10 min each by using lab detergent (SCAT 20-X, Japan), distilled water, acetone (Wako, Japan), isopropanol (Wako, Japan), and distilled water followed by drying and a plasma treatment (electronic Diener, Plasmasurface-technology, Germany) of 5 min, prior to the spin coating process. The aqueous poly(3,4-ethylenedioxythiophene) polystyrenesulfonate (PEDOT:PSS) (Clevious PVP AI 4083), which is a commonly used hole transport material,18,19 was first filtered using a 0.45 μm PVDF filter, and 100 μL was coated in ambient atmosphere condition at 4000 rpm for 50 s followed by heating treatment of 30 min at 175 °C to evaporate the water completely. Samples coated were then transferred immediately inside the glovebox equipped with constant N2 gas circulation (O2 < 0.1 ppm and H2O < 0.1 ppm), and the rest of the coating was done inside the glovebox to avoid the oxidation of Sn2+. The perovskite precursor solution, FA0.5MA0.5Sn0.5Pb0.5I3 (FAMA), was prepared as mentioned earlier.18 However, a higher concentration of precursor solution was prepared in our case to obtain a thickness of ∼620 nm (Figure S1a,b). At first, the FASnI3 (1.15 M) precursor solution was prepared by mixing 197.18 mg of formamidinium iodide (FAI, NH2CHNH2I) (TCI, Japan), 427.8 mg of tin iodide (SnI2) (Sigma-Aldrich, Japan), and 18.02 mg of tin fluoride (SnF 2 ) (Sigma-Aldrich, Japan) in dimethylformamide (Sigma-Aldrich, Japan)/dimethyl sulfoxide (Sigma-Aldrich, Japan) (800 μL/200 μL) and kept for 3−4 h at room temperature under constant stirring at 500 rpm. The

occurring within Sn−Pb absorber solar cell increases with higher Sn percentage which is mainly contributed by vacancy created19−21 due to oxidation of Sn2+. However, this increased Sn amount also changes the band gap and other electrical properties of the absorber layer, which in consequence changes its interaction with the ET and HT layers.22 TiO2 semiconductor has been conventionally implemented as ET layer for high-performing Pb-based solar cells so far in n−i−p configuration.23,24 Hence, initially TiO2 layer was selected for Sn−Pb based solar cells as well; however, it was later discovered by our group19 that the interface between TiO2 and Sn absorber layer creates new trap states which are large recombination centers, further contributing in Voc loss. To avoid these interfacial losses, another n-type material such as C60 has been reported,17−19 employing an inverted p−i−n solar cell design. In this work, we employed the inverted solar cell structure using a 50−50 mol % Sn−Pb absorber16−18 with incident photon-to-current efficiency (IPCE) touching 1060 nm feat, focused on increasing the Voc by the introduction of a novel spike structure strategy at Sn−Pb absorber and ET(C60) layer interface. The perovskite absorbers considered to have similar material properties with inorganic materials,25,26 such as Cu(In,Ga)Se2 (CIGS). In fact, the planar solar cell design is similar in both PSC and CIGS, typically transparent conductive oxide (TCO)/HT layer/perovskite/ET layer/metal contact and TCO/buffer/CIGS/metal contact, respectively. In case of CIGS solar cell, it has been proved that a conduction band offset (CBO) between the buffer layer and CIGS absorber layer is necessary to achieve higher PCEs.25 The CBO helps to reduce the carrier recombination at the interface, and as a result a higher Voc can be obtained. It has been theoretically proven by Minemoto et al.26 that a similar strategy can be used in PSC B

DOI: 10.1021/acs.nanolett.8b00701 Nano Lett. XXXX, XXX, XXX−XXX

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Figure 2. Photovoltaic parameters for 12 independent devices based on different concentration of PCBM, measured using an optical mask with an illumination area of 0.10 cm2. (a) PCE; (b) Voc; (c) JSC; (d) FF.

FASnI3 solution, as reported before27 when kept for long hours, slowly goes under self-oxidation to form Sn4+ ions in the precursor mixture; hence, it should be used for making films immediately after proper mixing. A MAPbI3 (1.63 M) precursor solution was prepared separately by mixing methylammonium iodide (MAI, CH3NH3I) (TCI, Japan) (181.38 mg), lead iodide (PbI2) (TCI, Japan) (526.01 mg), and lead thioscynate (Pb(SCN)2) (Sigma-Aldrich, Japan) (12.67 mg) in DMF/ DMSO (630 μL: 70 μL) under stirring for 3−4 h. Finally, 600 μL of FASnI3 and 400 μL of MAPbI3 were mixed to obtain FAMA precursor to deposit the absorber layer on the top of PEDOT:PSS layer. The absorber layer as shown in Figure 1 was formed in two-step spin coating utilizing an antisolvent approach; the perovskite solution (45 μL) was dropped and spun at 1000 rpm for 10 s, followed by 5000 rpm for 50 s. During the second spinning process, 750 μL of toluene (SigmaAldrich, Japan) as antisolvent28 was dripped at 11−15 s to remove solvent and to form an intermediate phase.29 As a final step, the coated film was sintered at 100 °C for 5 min to form highly dense and pinhole free films (Figure S2a), exhibiting the grain size of ∼400−500 nm. Fullerene (C60) (nanom purple SC) (20 nm), bathocuproine (BCP) (Sigma-Aldrich, 99%) (5 nm) and silver (Ag) (100 nm) were then sequentially deposited by thermal evaporation on the top of the perovskite layer, as shown in Figure 1b. The devices with phenyl-C61-butyric acid methyl ester (PCBM, Sigma-Aldrich, Japan) coating on top of FAMA were fabricated by spin coating the PCBM solution prepared in o-dichlorobenzene (Sigma-Aldrich, Japan). A volume of 80 μL was dropped on the top of FAMA and spun after 5 s at 2000 rpm for 60 s; later samples were covered with Petri dish for 12 h at room temperature for solvent annealing before deposition of C60. Results and Discussion. The thin films were characterized by X-ray diffraction (XRD) analysis (RINT-Ultima III, Rigaku, Japan) as depicted in Figure S3. A perusal of XRD pattern for FASnI3 and MAPbI3 in Figure S3a,b confirms their formation as

reported earlier.14−18 FASnI3 exhibits the orthorhombic (Amm2) crystal structure, confirmed by the formation of (113) plane at 24.3°. MAPbI3 shows a tetragonal (I4cm) crystal structure as indicated by the existence of strong diffraction peaks at 23.44° and 24.46°, which are derived from the (211) and (202) planes of the tetragonal perovskite structure. As a result, FAMA formed by mixing of MAPbI3 and FASnI3 shows a pattern similar to orthorhombic crystal structure as noticed by the evolution of (113) plane. Also, according to Figure S2b, FAMA exhibits strong diffraction peak of (110) and (220) planes, which reveals the formation of highly oriented and crystalline thin film. The complete solar cell ITO/PEDOT:PSS/FAMA/PCBM/ C60/BCP/Ag, fabricated is shown in Figure 1b, and different layers with their respective thickness are reported. It is important to mention here that the bilayer phenyl-C61-butyric acid methyl ester/fullerene (PCBM/C60), is commonly used as ET layer in the p−i−n design of MAPbI3 based PSCs.12,23 However, it is very important to note down that lowest unoccupied molecular orbital (LUMO) of PCBM (−4.0 ∼ −4.3 eV)21,30 is lower than that of MAPbI3(−3.7 ∼ −4.0 eV),3−10 as mentioned in various reports earlier. Therefore, PCBM/C60 gives a perfect energy cascading to the electron transport and reduces the recombination loss at the interface. Furthermore, C60 is helpful in the extraction of the electrons because of high electron mobility (1.6 cm2 V s−1).31 In case of solar cells reported, with FAMA17,18 as absorber layer the absence of PCBM could be due to the lower LUMO level (−4.17 eV) of it, which logically seems to provide a barrier to electron transfer from absorber layer to C60. Therefore, it becomes very important to know correctly the energy levels of each layer for appropriate selection of the different layers in solar cell designing. The absorption spectrum of FAMA and PCBM was observed as shown in Figure S4 by UV−vis−NIR spectrophotometer (V-570, JASCO, Japan) with an accuracy of ±1.5 nm. The optical instrument is equipped with the doubleC

DOI: 10.1021/acs.nanolett.8b00701 Nano Lett. XXXX, XXX, XXX−XXX

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Nano Letters Table 1. Photovoltaic Parameters of the Devices under Study PCE (%) Voc (V) Jsc (mA/cm2) FF

C60

1PCBM-C60

5PCBM-C60

10PCBM-C60

20PCBM-C60

12.93 ± 1.25 0.66 ± 0.01 27.15 ± 1.67 0.72 ± 0.02

15.69 ± 0.65 0.71 ± 0.01 29.20 ± 0.62 0.76 ± 0.01

16.73 ± 0.54 0.73 ± 0.01 30.29 ± 0.60 0.75 ± 0.01

15.48 ± 1.01 0.71 ± 0.01 29.23 ± 1.56 0.74 ± 0.01

13.94 ± 1.20 0.71 ± 0.01 28.85 ± 1.82 0.68 ± 0.02

Figure 3. (a) I−V curves under simulated 1 sun of solar cell without (black) and with (red) PCBM layer (∼25 nm). Inset shows a table with the photovoltaic parameters in forward and reverse direction measurement. (b) IPCE as a function of monochromatic wavelength recorded for solar cells without (black) and with (red) PCBM layer (∼25 nm); also shown are the integrated Jsc of 26.16 mA/cm2 obtained for C60 and Jsc of 28.40 mA/cm2 for 5PCBM-C60 from the respective IPCE spectra. (c) Steady-state output of 5PCBM-C60 at maximum power point (0.61 V) of the champion device under constant illumination of 1 SUN.

istics were evaluated with a solar simulator (KHP-1, BunkoKeiki, Japan) equipped with a xenon lamp (XLS-150A). The light intensity was checked and adjusted to AM 1.5 (100 mW/ cm2) by spectroradiometer (LS-100, Eiko Seiki, Japan). PSC without PCBM on ET layer side was named as C60 and PSCs with different PCBM concentrations as 1PCBM-C60, 5PCBMC60, 10PCBM-C60, and 20PCBM-C60 when absorber layer was coated with 1, 5, 10, and 20 mg/mL of PCBM in odichlorobenzene, respectively. The thickness of PCBM (20 mg/mL) layer coated on the top of perovskite layer was estimated to be ∼100 nm by white light interferometric microscope (Nikon Eclipse LV 150, Japan) (Figure S6) and thicknesses were approximately assumed for less concentrations accordingly. Figure 2 and Table 1 summarize the photovoltaic performance of 12 independent devices for each concentration of PCBM layer. The device characteristics were measured with an optical mask, illuminating an area of 0.10 cm2. The best PCE of C60 device, that is, without PCBM layer, was around 14.18% with a very less hysteresis, which approves the formation of good quality films and interfacial contact. It is clearly seen that average PCE of the C60 improved from 12.93 ± 1.25% to

beam system with single monochromator in the wavelength 190−2500 nm with a halogen lamp (300−2500 nm) as a light source. Thin films of both PCBM and FAMA were directly deposited on the glass surface to exactly determine the band gap. PCBM showed a band gap of ∼2 eV, which is in agreement with the various reports.20,21 In case of FAMA, a band gap of 1.25 eV was evaluated considering a direct band gap, which is an ideal band gap to achieve the high power conversion efficiency (PCE) according to the ShockleyQueisser limits.23 The highest occupied molecular orbital (HOMO) of FAMA and PCBM was determined using photoelectron yield spectroscopy (PYS) (BUNKOKEIKI, KV205-HK), as shown in Figure S5, was measured to be at −5.4 and −6.0 eV respectively. As a result, the LUMO values thus obtained were calculated to be −4.15 and −4.0 eV for FAMA and PCBM, respectively, as drawn in Figure 1c, resulting in a CBO of ∼0.15 eV and providing a spike between absorber and ET (C60) layers. The PSC with and without PCBM layer was fabricated for understanding the effect of spike provided by PCBM layer in between FAMA and C60. The current−voltage (I−V) characterD

DOI: 10.1021/acs.nanolett.8b00701 Nano Lett. XXXX, XXX, XXX−XXX

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Figure 4. (a) TRPL measured at 980 nm for FAMA; FAMA-C60, FAMA film coated with C60; FAMA-5PCBM-C60, FAMA film coated with 5 mg/ mL of PCBM and C60; FAMA-20PCBM-C60, FAMA film coated with 20 mg/mL of PCBM and C60. (b,c) ns- TA of FAMA-C60 and FAMA5PCBM-C60, pumped at 470 nm and probed at 1300 nm. τr is the recombination time obtained after exponential fitting. (d) Schematic showing the charge extraction and recombination processes without and with PCBM layer. τf shows the forward injection time from FAMA to C60.

more than ∼28 mA/cm2, after utilizing PCBM layer coating in 5PCBM-C60, which represents a reasonable correlation with Jsc measured under 1 sun. The integrated Jsc from IPCE, as estimated in Figure 3b, is little lower than that obtained from J−V characteristics. The possible reason for this discrepancy can be due to lower illumination intensity during IPCE measurement, where recombination processes are more prominent because of trap states and space charge effects. However, the difference is within the limit of ∼7%, which is a close match, as explained by Zimmermann et al.32 Also, it would pay particular attention that absorption reached the wavelength of 1060 nm which confirms the band gap of ∼1.25 eV for FAMA, as observed previously in Figure S4a. From Figure 2 and Table 1, we further conclude that 10PCBM-C60 and 20PCBM-C60 devices with deposition of high concentration of PCBM led to decrease in overall solar cell performance, which could be answered by an increase in resistance to electron flow provided by increased thickness of PCBM, resulting in loss of FF and PCE. Moreover, variation in solar cell performance with PCBM thickness can be seen as the effect of electron diffusion length or electron mobility of PCBM. For effective transport of electrons, it has been welldefined that the electron diffusion length should be comparable to the thickness of the material.9 Therefore, a decrease in performance of 10PCBM-C60 and 20PCBM-C60 can be attributed to higher thickness compared to electron diffusion length inside PCBM. Charge Transfer Analysis. To further illustrate the physical mechanism behind the introduction of spike energy level using PCBM, time-resolved PL (TRPL) measurements were performed using the compact NIR photoluminescence lifetime spectrometer C12132 (HAMAMATSU, Japan) as shown in Figure 4a and the corresponding PL spectra are shown in Figure S6. A laser beam of wavelength 532 nm with

15.69 ± 0.65% for 1PCBM-C60. The main parameter which improved was Voc from 0.66 ± 0.01 to 0.71 ± 0.01 V, indicating the decrease in potential loss. This increase in Voc can be attributed to the spike structure effect due to the introduction of PCBM layer on the top of the absorber layer. Also, it is interesting to note that fill factor (FF) was improved from 0.72 ± 0.02 to 0.76 ± 0.01, suggesting a decrease in series resistance and facile electron transfer. This improvement in FF was also proved by simulation analysis,25,26 as the result of spike structure formation on the absorber layer. It is worth to be noted that after PCBM deposition on absorber layer, the shortcircuit current density (Jsc) was also improved from 27.15 ± 1.67 to 29.20 ± 0.62 mA/cm2, as a result of enhancement in IPCE which could be due to surface passivation by PCBM. The improvement of all photovoltaic parameters illustrated in Figure 2 and Table 1 are observed for the devices with PCBM layer, regardless of the PCBM concentration. The highest PCE of 17.6% was obtained in case of 5PCBM-C60 with Jsc, Voc, and FF of 30.56 mA/cm2, 0.75 V, and 0.76, respectively, in the forward scan (at 10 mV s−1 scan rate) as reported in Figure 3a. Similarly, a very less hysteresis of 5PCBM-C60 (inset table of Figure 3a) proves the reliability of the device measured. The reliability of the high-performing device was also checked by forward biasing the device at maximum power point voltage of 0.61 V as presented in Figure 3c and it showed the efficiency of ∼17.5% after 200 s, which is in agreement to the PCE obtained from I−V curve. To confirm the accuracy of Jsc measured under 1 sun, devices were checked by IPCE measurement as well. The IPCE was measured under a constant photon flux of 1 × 1016 photon per cm2 at each wavelength in direct current mode using the action spectrum measurement system connected to the solar simulator (CEP2000, Bunko Keiki, Japan). As depicted from Figure 3b, a photocurrent of ∼26 mA/cm2 in case of C60 was increased to E

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Figure 5. (a) Nyquist plot under dark of C60 device (black dots) and 5PCBM-C60 (red dots) at short circuit condition. (b) Capacitance measured under dark at short circuit condition. (c) Resistance fitted in a range of voltages applied.

reported earlier,33 where photogenerated electrons can tunnel the polymer or insulating layer. However, it is worth noticing the difference between tunneling effect and the spike structure concepts in the present case, where thickness of PCBM layer is ∼25 nm (approximately estimated by using optical microscope in Figure S8) for best-performing device (5PCBM-C60) and the photogenerated electrons cross the potential barrier due to thermal energy gained at room temperature. To probe the difference between tunneling effect and CBO effect in the present case, the dependence of solar cell performance with an increase in CBO becomes very important. Therefore, a high LUMO energy level (3.7 eV)34 based fullerene derivative indene-C60 bisadduct (ICBA) was selected with a potential barrier of 0.45 eV (Figure S9). A thickness (∼23 nm, estimated approximately from the thickness as measured in Figure S10) equal to the PCBM layer (in 5PCBM-C60) was coated on perovskite absorber (FAMA) layer, utilizing a concentration of 5 mg/mL. The solar cell 5ICBA-C60 was fabricated, keeping the other layers similar to 5PCBM-C60, and in the result the device exhibited drastically poor Voc (0.48 V) with PCE of 0.96% as depicted in Figure S11. This confirms that there is no possibility of tunneling in both the cases i.e PCBM or ICBA and the enhancement of PCE by PCBM layer introduction is solely due to spike energy level of PCBM. At this point of discussion, it becomes clear that PCBM is providing the CBO to FAMA and playing the similar role as a buffer layer in CIGS. According to a previous report,35 while discussing the critical role of the buffer layer in CIGS, modification of surface chemistry of absorber layer leading to reduced trap states has been mentioned, this can be considered as passivation by the buffer layer. Therefore, PCBM in the present case is playing the role of passivation as well apart from spike structure which is similar to the previous observation.36 To further understand the role of the PCBM layer insertion between perovskite absorber and ET layer, electrochemical impedance spectroscopy (EIS) measurements were carried out. Such a technique has been widely used for the characterization of perovskite devices mainly to detect several electrical processes that involved the charge dynamics at different time scales in the solar cells.37,38 Here, two different solar cells were analyzed by EIS under dark, without and with PCBM layer, C60, and 5PCBM-C60, respectively. A small AC perturbation

0.47 mW power was used to excite the samples. Table S1 summarizes the excited carrier lifetime values of radiative recombination occurring inside FAMA absorber layer without C60, with C60 (FAMA-C60), with 5PCBM/C60 (FAMA5PCBM-C60), and with 20PCBM/C60 (FAMA-20PCBMC60). PL quenching and faster TRPL decay with the incorporation of ET layers were observed as expected,21 which is the signature of improved electron transfer with ET layer. Furthermore, it was noticed that with 5PCBM/C60 and 20PCBM/C60, the carrier lifetime becomes a bit shorter compared to FAMA only; however, this difference is not very large, which indicates an approximately same forward electron injection rate from FAMA to ET layers. To clarify the exact role of PCBM in enhancing the solar cell activity, ns-TA (nanosecond transient absorption) was also performed to study the charge recombination dynamics at the interface between FAMA and ET layers. The TA setup consists of a pump light source, OPO (Surelite II-10FP) excited by an Nd:YAG pulse laser (Panther, Continuum, Electro-Optics Inc.). We used pulsed light to excite the samples with a wavelength of 470 nm and samples were probed at a wavelength of 1300 nm. FAMA samples without ET layers exhibited no absorption signals when probed at 1300 nm (Figure S7). For FAMA-5PCBM-C60 as depicted in Figure 4c, the result implies a decrease in carrier recombination rate using PCBM insertion with an increase in carrier recombination lifetime (τr) from 91.4 ms (FAMA-C60, Figure 4b) to 142.8 ms, which supports the enhancement of Voc and Jsc for 5PCBMC60 devices. To draw the conclusion from charge dynamics measured above, a schematic in Figure 4d is drawn to understand the role of spike structure in complete device. It shows that absence of PCBM layer results in higher recombination at the interface. In previous reports,25,26 it has been observed that a cliff does not impede the photogenerated electrons; however, it lowers the activation energy for carrier recombination at the interface, which results in a decrease of Voc. On the contrary, when a PCBM film was inserted between the absorber and C60 layer, a spike was introduced (Figure 4d), which increases the activation energy for carrier recombination at the interface, improving the Voc and overall PCE. Increase in solar cell performance with the passivation of perovskite absorber layer surface by a thin polymer layer has been F

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with an amplitude of 20 mV was applied using a high sensitivity potentiostat (AUTOLAB PGSTAT204) connected to impedance module (FRA32M) from zero bias to open circuit bias every 100 mV. Each voltage varied in a range of frequencies from 1 to 100 MHz. At short circuit conditions, the impedance spectra showed in Figure 5a displays one semicircle in the Nyquist plot. A large semicircle is clearly observed in the impedance spectrum when a small amount of PCBM was added between the FAMA absorber and C60 layers compared to the small semicircle of the C60 solar cell. Therefore, the pristine C60 solar cell showed large electrical barrier that is reduced by the addition of the PCBM layer, and consequently, the charge carrier transport to C60 layer is more efficient as it is reflected in the Jsc results (see Table 1). Additionally, 5PCBM-C60 device exhibited slightly less interfacial capacitance (Figure 5b), 1.6 × 10−7 Fcm−2, at low-frequency region and at short circuit conditions, being 2.4 × 10−7 Fcm−2 for C60. Slightly less ionic accumulation at the surface of FAMA absorber layer was found by the addition of PCBM layer and a reduction of the traps sites at the interface was detected in the 5PCBM-C60 solar cell. The EIS spectra can be well-fitted using the equivalent circuit model reported previously.39 Figure 5c displays the fitted resistance at high-frequency region that corresponds to the transport resistance of electron along the perovskite layer interface.40 At constant bias applied, the 5PCBM-C60 devices showed larger resistance than that to C60 solar cell, indicating less trap recombination center at the interface FAMA/C60 by the addition of PCBM layer. The charge carrier can be efficiently extracted by the addition of the PCBM layer due to better contact between FAMA absorber and C60 layers. Conclusions. In summary, the formation of a spike structure inbetween the absorber layer (FAMA) and ET (C60) layer was introduced as one of the approaches to reduce the potential loss in Sn−Pb mixed PSCs. A thin layer of PCBM was selected, which was exhibiting a higher CBP (∼0.15 eV) compared to FAMA and resulted in an increase of Voc to 0.75 V with best solar cell performance of 17.6%. The role of PCBM in reducing the recombination center at the interface was proved by TRPL, ns-TA, and EIS, which is a direct effect of CBO provided by PCBM. Moreover, it was demonstrated that improved efficiency can be realized by doing interfacial engineering, even if the absorber layer is same. We feel that our finding in the report can broaden the understanding of PSCs, and spike structure-based solar cell configuration could be a possible solution to increase the Voc through interfacial engineering.

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Gaurav Kapil: 0000-0002-5066-4978 Jakapan Chantana: 0000-0002-5711-4979 Qing Shen: 0000-0001-8359-3275 Taro Toyoda: 0000-0002-2067-3689 Takurou N. Murakami: 0000-0002-2066-3982 Notes

The authors declare no competing financial interest.

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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.nanolett.8b00701. Thickness measured for different layers of the solar cell, XRD of FAMA, PYS of FAMA and PCBM, UV−vis spectra of FAMA and PCBM, PL data of FAMA with different ET layers, energy diagram using ICBA, I−V curve with ICBA, table of fitted TRPL data (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. G

DOI: 10.1021/acs.nanolett.8b00701 Nano Lett. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.nanolett.8b00701 Nano Lett. XXXX, XXX, XXX−XXX