Systematically Optimized Bilayered Electron Transport Layer for

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Systematically Optimized Bilayered Electron Transport Layer for Highly Efficient Planar Perovskite Solar Cells (# = 21.1%) Seulki Song, Gyeongho Kang, Limok Pyeon, Chaesung Lim, Gang-Young Lee, Taiho Park, and Jongmin Choi ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.7b00888 • Publication Date (Web): 25 Oct 2017 Downloaded from http://pubs.acs.org on October 25, 2017

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ACS Energy Letters

Systematically Transport

Optimized

Layer

for

Bilayered

Highly

Efficient

Electron Planar

Perovskite Solar Cells (η = 21.1%) Seulki Song,a‡ Gyeongho Kang,a‡ Limok Pyeon,a Chaesung Lim,a Gang-Young Lee,a Taiho Parka*and Jongmin Choib* a

Seulki Song, Gyeongho Kang, Limok Pyeon, Chaesung Lim, Dr. Gang-Young Lee, Dr. Taiho

Park Chemical Engineering, Pohang University of Science and Technology (POSTECH), 77 Cheongam-Ro, Nam-gu, Pohang, Kyoungbuk, Korea. E-mail: [email protected] b

Dr. Jongmin Choi

Department of Electrical and Computer Engineering, University of Toronto, 10 King’s College Road, Toronto, Ontario M5S 3G4, Canada. E-mail: [email protected]

‡ These authors contributed equally to this work.

ABSTRACT

ACS Paragon Plus Environment

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Understanding and controlling interfacial charge transfer at heterojunction of optoelectronic devices are currently receiving extensive interest. Here, we study the parameters that can influence the electron extraction in planar perovskite solar cells (P-PSCs) using spin-coated SnO2 and TiO2, anodized-TiO2, and bilayered electron transport layers (ETL) composed of SnO2 and TiO2 or SnO2 on a-TiO2 (SnO2@a-TiO2). These varied free energy difference (∆G) values between the ETL and perovskites, electron mobility (µe) of the ETL, and quality of physical contact between the ETL and fluorine-doped tin oxide (FTO). Among the various ETLs, the bilayered ETL (SnO2@a-TiO2) gives a large ∆G as well as defect-free physical contact. The resulting P-PSC exhibits a PCE of 21.1% and stabilized efficiency of 20.2% with reduced hysteresis. This result emphasizes that a large free energy difference (∆G) value plays an important role in electron extraction. More importantly, the defect-free physical contact is also crucial for achieving improved electron extraction. TOC GRAPHICS

Typical planar perovskite solar cells (P-PSCs) comprising an n-i-p structure are made up of a counter electrode (anode)/hole transport layer (HTL)/perovskite layer/electron transport layer


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(ETL)/working electrode (cathode).1 It has been reported that the electron (µe) and hole (µh) mobilities of perovskites are very high, exhibiting a range of 101–102 cm2/Vs.2-8 The ability of the ETL and HTL to effectively extract photo-induced electrons and holes from perovskites and transfer them to the anode and cathode, respectively, is therefore of importance when improving photovoltaic performances.9, 10 Especially for P-PSCs without a mesoporous layer, the intrinsic nature of the ETL is becoming more important. Currently, spin-coated TiO2 ETLs (without any dopants) prepared by the sol-gel process are widely used; P-PSCs in which they are used generally exhibit efficiencies in the range of 10.2%–16.1% (the exact value depends on the type of HTL and perovskite used).1, 11-16 Since Miyasaka et al. and Yan et al., independently, applied a spin-coated SnO2 ETL to PPSCs,17,18 SnO2 ETLs have become one of the major ETLs for P-PSCs. For example, it was independently demonstrated by both Yan et al. and ourselves that PCEs of over 17% can be achieved in P-PSCs employing SnO2 ETLs, probably owing to the high µe (approximately 10−3 cm2/Vs)18 compared with TiO2 (approximately 10−5 cm2/Vs)19 as well as their well-matched energy levels. However, the PCE values obtained from P-PSCs were still lower than that of the corresponding mesoporous PSCs (M-PSCs) (e.g. TiO2 ETL-based M-PSCs, which have a PCE of approximately 22%),20 these cells demonstrate highly efficient electron extraction owing to the large contact area between the mesoporous metal oxide scaffold layer and the perovskites.2124

Therefore, to realize the similar high efficiency of M-PSCs, it is important to understand the

parameters that can influence the charge extraction, transport, and collection for obtaining highefficiency P-PSCs. Some groups have reported on the parameters that could influence the electron extraction. For example, Katoh et al. reported that the relative efficiency of the electron injection increases gradually with increasing change in free energy (∆G).25 In addition, we also


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reported a relation between charge transport and ∆G in dye-sensitized solar cell systems.26, 27 Sakata et al. reported the relationship between decay rates in time-resolved photoluminescence (TRPL) and the difference in the conduction band edge of dyes and semiconductors.28 Kamat et al. also reported the relationship between the energy level of quantum-dot-sensitized metal oxide systems and the electron transfer rate.29 In addition, the relationship between the charge extraction ability and conductivity was also investigated by Bulović et al. They showed that the quenching of excitons generated in a tris-(8-hydroxyquinoline) aluminium (Alq3) layer increased with increasing conductivity of the SnO2 films.30 Recently, Hagfeldt et al. reported that a layer of SnO2 showed a high performance in P-PSCs due to its deeper conduction band.31 However, due to the structure and the photocurrent generation mechanism in P-PSCs being dissimilar to other photovoltaics, more specific and detailed investigations are required for P-PSCs, and studies on parameters influencing electron extraction have been rarely conducted at the interfaces of perovskite/ETL/fluorine-doped tin oxide (FTO).

Figure 1. Schematic illustration of electron extraction in an n-i-p-type planar device. Electron extraction occurs from the perovskite to the FTO via injection, transportation, and collection. In P-PSCs comprising an n-i-p structure, we supposed that the electron extraction process can be divided into three steps: electron injection from the perovskites to the ETL, electron transportation through the ETL, and electron collection from the ETL to the FTO as illustrated in Figure 1. Assuming that there are no physical defects between the ETL and a highly rough FTO


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layer, these might be affected by the ∆G between the conduction band edges of the ETL and perovskites and µe of the ETL when other conditions are identical except ETLs; although the purity of the materials, molecular packing at the interfaces, or environmental conditions such as temperature and moisture could be considered, these could not be the main parameters affecting electron extraction processes under identical experimental conditions.32 Therefore, to gain a deeper understanding for the electron extraction processes, first we try to confirm the effects of ∆G and µe of the ETL using four different types of ETLs: spin-coated single SnO2, TiO2 ETLs, bilayered SnO2@TiO2, and TiO2@SnO2 ETLs on FTO (see Figure 2a for details). We further extend our investigation to determine the issue of physical contact using defect-free anodized TiO2 (a-TiO2) (denoted as SnO2@a-TiO2; see Figure 3 for details) instead of spin-coated TiO2. Herein, we demonstrate the importance of ∆G between the perovskites and the ETL (rather than µe of the ETL) and defect-free physical contact between the ETL and FTO. The resulting P-PSC employing a SnO2@a-TiO2 ETL exhibits a 21.1% J–V scan efficiency and a 20.2% maximum power point (MPP) efficiency with reduced hysteresis in such a planar structure. The conduction band minimum (CBM) and work function (WF) values of the four different types of ETLs, FTO and perovskite were derived from the valence band maximum (VBM) values determined using ultraviolet photoelectron spectroscopy (UPS) and bandgap (Eg) values determined using UV–vis. absorption spectroscopy (Figure S1 and Figure S2). From an energy point of view, the driving force (∆G) for electron injection in a p-n semiconductor junction can be expressed as (Table S1):25-30

Perovskite ETL ∆G = E CBM − E CBM

(1)


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Figure 2. (a) Illustration of ETLs with different free energies (∆G) and electron mobilities (µe). Case 1: SnO2 (larger ∆G with higher µe) and TiO2 (smaller ∆G with lower µe). Case 2: SnO2@TiO2 (larger ∆G with similar µe) and TiO2@SnO2 (smaller ∆G with similar µe). (b) Histograms of devices fabricated by TiO2, SnO2, TiO2@SnO2, and SnO2@TiO2 ETLs. Perovskite ETL is the CBM of perovskite, and E CBM is the Where, ∆G is the free energy difference, E CBM

CBM of the ETL. Meanwhile, the µe values of the four different types of ETLs were estimated from measurements of the space-charge-limited current (SCLC) (Figure S3). Figure 2a shows the summarized illustrations of the energy levels, ∆G, and µe for the four different types of ETLs. The ∆G for the SnO2 ETL was approximately 0.60 eV, greater than that (~0.24 eV) of the TiO2 ETL due to the deeper CBM. In addition, µe for the SnO2 ETL was 1.19 × 10-3 cm2/Vs, almost two orders of magnitude higher than that (1.59 × 10-5 cm2/Vs) of the TiO2 ETL, consistent with the literature.18 To evaluate the electron extraction efficiency of spin-coated single SnO2 and TiO2 ETLs from perovskites, individual devices for each ETL were fabricated. Photovoltaic parameters of P-PSCs with different ETLs could depend on many factors such as the film thickness, coverage of ETLs on FTO and contact with perovskite. Therefore, we firstly compared the high-resolution scanning electron microscope (HR-SEM) images of the perovskites (Figure S4). The perovskites on TiO2, SnO2, and bilayer SnO2@TiO2 showed very similar grain sizes without pinholes. Thus, we considered that the Shockley-Read-Hall (SRH) recombination at the


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perovskites of all the devices similarly occurred. In addition, we employed highly optimized thicknesses of the ETLs for each device. We found that the P-PSCs with thicknesses of 30~50 nm (SnO2), average ca. 40 nm (TiO2),19 and average ca. 50 nm (bilayered ETLs) exhibited the highest photovoltaic performances (see Figure S5 for the cross-sectional SEM images of bilayered ETLs). Therefore, the differences in the photovoltaic performances could be mainly ascribed to the nature of the ETLs rather than the film thickness, coverage of ETLs on FTO or contact with perovskites. The short circuit current (JSC) value is closely related to the electron collection efficiency, which is formulated as the following equation; JSC = q∫ bs(E) QE(E) dE, where bs(E) is the incident spectral photon flux density, QE(E) dE is the number of photons in the energy range from E to E + dE, and q is the electronic charge. The quantum efficiency (QE) depends on the light harvesting efficiency, which is related to the charge transport and extraction efficiency in the device.33 The trend of the PCE was very similar to that of JSC. This indicate that difference of PCEs is mainly attributed to charge extraction ability in P-PSC. The overall photovoltaic performances of the SnO2 devices were superior to the TiO2 devices, as shown in the histograms of the photovoltaic parameters (Figure 2b). Apart from the JSC values, VOC values of SnO2 devices much higher than the others despite of higher CBM of TiO2. The Fermi energy level of the ETL is determined by the electron extraction ability from the perovskite layer, which plays an important role in determining the VOC rather than the CBM of the ETL.34 However, in the case of SnO2, it can be inferred that the ETL has a higher VOC value because the charge accumulation at the interface is reduced due to efficient electron injection from the perovskite, and the decrease of the VOC is relatively reduced. The overall results account for the advantage of greater ∆G, as


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well as the higher µe of the SnO2 ETL in the P-PSCs. Note that the PCE values for planar devices employing a single ETL of TiO2 (13.1%) or SnO2 (19.5%) are in good agreement with those reported previously by our group (12.5% for TiO2 ETLs) and Yan (17.4% for SnO2 ETLs).18, 19 However, it was still not clear which of the two parameters, ∆G and µe, is more crucial, or if both these parameters contribute to higher photovoltaic performance. Thus, it was essential that each parameter be independently evaluated. For this purpose, a device geometry comprising bilayered ETLs such as SnO2@TiO2 and TiO2@SnO2 ETLs on FTO was used, and their electronic transportation properties (i.e., µe value) were evaluated (see Figure S1, Figure S2, and Figure S3 and Table S1, and Table S2). The ∆G values of SnO2@TiO2 and TiO2@SnO2 ETLs were same with those of SnO2 and TiO2 ETLs, respectively. The overall charge transportation rate in the multilayers might be governed by one specific layer having the lowest charge mobility among the many layers. Indeed, the µe values of SnO2@TiO2 and TiO2@SnO2 ETLs were in the range of 1.27–1.37 10-5 cm2/Vs (Figure S3), which is two orders of magnitude lower than that of a single SnO2 ETL, but similar to the TiO2 ETL. The overall photovoltaic performances of the SnO2@TiO2 devices are higher than those of TiO2 and TiO2@SnO2 devices, but lower than SnO2 devices (Figure 2b). This indicated that the greater ∆G by direct contact of SnO2 and the perovskite, rather than the higher µe in the ETL, was critical to achieve high PCEs in P-PSCs. As stated above, the devices with a large ∆G exhibited better electron extraction efficiencies when there were no physical defects at the FTO/ETL interface. However, the FTO retains a highly rough surface; thus, the quality of physical contact at the FTO/ETL interface should be considered to obtain a high electron extraction efficiency. Recently, we revealed that there are many physical defects in metal oxide ETLs, e.g. TiO2 prepared by the spin-coated sol-gel process. These defects were caused by the high surface roughness of the FTO, volume shrinkage


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(e.g. from Ti(O–iPr)4 to TiO2) in the sol-gel process (Figure 3a), and the discrepancy between the surface energy of the FTO and the metal oxide (Figure S7). These physical defects led to a reduction in the effective physical contact area between the FTO and ETL (at the void areas between them), partial shortage (at protruding areas), high back reaction (at areas that were very thin), and inefficient electron transport (at areas that are very thick).

Figure 3. (a) Schematic illustration of the bilayered SnO2@a-TiO2 ETL. The dotted red circles indicate examples of physical defects on the SnO2 ETL (also see Figure S6 for a vertical high-resolution transmission electron microscope (HR-TEM) image). (b) Illustration of bilayered ETLs with large ∆G and moderate µe. High-resolution scanning electron microscope (HR-SEM) and HR-TEM images. (c) Top-down SEM images of bare FTO, a-TiO2 on FTO, and spin-coated SnO2 on an a-TiO2/FTO substrate. (d) HR-TEM cross-sectional images of SnO2@TiO2 ETL and corresponding Sn and Ti mapping images obtained from electron energy loss spectroscopy analysis. Interestingly, the PCE of the P-PSC was improved by 20.7% after the Hagfeldt group used the atomic layer deposition (ALD) and chemical bath deposition (CBD) methods. These ALD and CBD methods allow for the deposition of dense ETLs, which in turn can enhance the physical contact between the ETL and the FTO.35 Thus, we introduced a physical defect-free a-


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TiO2 layer on a highly rough FTO instead of spin-coated TiO2, following our previous report.19 Then, an additional spin-coated SnO2 layer was deposited, giving a bilayered SnO2@a-TiO2 ETL. Here, we note that we tried to introduce a well-defined thin anodized SnO2 layer, but failed to obtain it, due to thermal instability of Tin at high temperature during the vacuum evaporation.35 UPS and SCLC measurements of a-TiO2 and SnO2@a-TiO2 ETLs were performed to obtain the intrinsic properties of the a-TiO2 layer (Figure S8). Thus, we could clarify the intrinsic properties of SnO2@a-TiO2 with ∆G = 0.56 eV (which is almost same with the ∆G of SnO2) and µe = 1.71 × 10−4 cm2/Vs (see Figure 3b). The CBM of a-TiO2 was 4.19 eV, which was almost identical to that of TiO2, but showed a greater shift to the vacuum level as compared to that of SnO2. The µe of a-TiO2 was one order of magnitude higher than that of TiO2, but lower than that of SnO2 (Table S3). In HR-SEM top-down images, the a-TiO2 layer retained the surface roughness of bare FTO owing to the formation of both a uniform film thickness and defect-free physical contact with the rough FTO substrate (Figure 3c). The surface of the SnO2 after spin coating the precursor solution followed by thermal annealing of the precursor film became smoother, but it still retained its roughness owing to its good wettability (e.g. similar surface energies as SnO2 and aTiO2 ETLs). High-resolution transmission electron microscope (HR-TEM) cross-sectional images of SnO2@a-TiO2 ETLs, combined with an electron energy loss spectroscopy (EELS) analysis, clearly demonstrated the existence of the a-TiO2 ETL with a uniform film thickness and defect-free physical contact between the very rough FTO and SnO2 layers (Figure 3d). To demonstrate the electron extraction capability of the ETLs, we fabricated P-PSCs and analysed their photovoltaic performances. The thickness of each ETL was very important, as we reported previously,19 and thus, we first optimized the thickness of the SnO2 ETL to be, on average, 30


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and 50 nm. Although the P-PSC with an average thickness of 50 nm exhibited a slightly higher photovoltaic performance than that with an average of 30 nm, the difference was not critical (Figure S9 and Table S4). The energy level of two ETLs in a bilayered ETL should be considered as well. The conduction band energy level of a-TiO2 is higher than that of SnO2. To overcome such an energy level imbalance as well as the electron flow bottleneck effects caused by the a-TiO2 ETL, we employed a very thin a-TiO2 layer of ~10 nm.10 After optimization, we found that the P-PSC with a thickness of ~40 nm (comprising an average of 30 nm-thick SnO2 and ~10 nm-thick aTiO2) exhibited the highest photovoltaic performance (Figure S10 and Table S4; also see Figure S11 for a cross-sectional HR-SEM image of an optimized P-PSC).

Figure 4. (a) Histogram of devices fabricated by a-TiO2, SnO2, and SnO2@a-TiO2. (b) Current–voltage (J–V) curves of each of the champion devices. (c) Stabilized power output and current at maximum power point as a function of the time for the champion cell. Forward and reverse J–V scan of (d) a-TiO2, (e) SnO2, and (f) SnO2@a-TiO2.


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For in-depth analysis, we compared three types of ETLs (a-TiO2, SnO2, and SnO2@a-TiO2) (See Figure. S12 for the morphology of a-TiO2 and SnO2 ETLs). As shown in Figure 4a, the overall PCEs, including the JSC, fill factor (FF), and open-circuit voltage (VOC) values, of the SnO2@a-TiO2 devices, were superior to those of the SnO2 and a-TiO2 devices. The best SnO2@a-TiO2 device had JSC = 22.9 mAcm−2, VOC = 1.20 V, and FF = 76.4%, and this yielded a PCE of 21.1% (Figure 4b); this value was much greater than that of the a-TiO2 and SnO2 devices (PCE = 14.0% for a-TiO2 and PCE = 19.0% for SnO2). For comparison, we also fabricated the device with CBD-treated SnO2 on spin-coated SnO2 following the method reported by Hagfeldt et al.35 (Figure S13). Resulting device showed a PCE of 19.6% and was lower than the device employing SnO2@a-TiO2 ETL. To the best of our knowledge, this is the highest efficiency in planar perovskite solar cells. Especially, physical contact without defects and excellent electron extraction efficiency due to ∆G gave very high VOC values of 1.2 V. The MPP efficiency at each MPP, which is generally accepted as the most reliable way to evaluate device performance,37-39 was 20.2% for the SnO2@a-TiO2 device (Figure 4c). This value also exceeded those obtained from the a-TiO2 and SnO2 devices (12.8% and 17.8%, respectively, see Figure S14 for the incident photon to current efficiency (IPCE)). Interestingly, the SnO2@a-TiO2 device showed much less hysteresis behavior than the others, as shown in Figure 4d–f (see Supplementary Table 5 for the photovoltaic parameters), due to the improved interfacial properties (i.e., the physical defect-free interface and the greater ∆G). In turn, it proposed there was still few physical defects in our SnO2@a-TiO2 device, especially at the interface between the a-TiO2 (still rough) and the spin-coated SnO2.


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Figure 5. (a) Normalized time-resolved photoluminescence (TRPL) decay of the perovskite on glass, a-TiO2, SnO2, and SnO2@a-TiO2 ETL samples. (b) Magnified graph of time range from 0 ns to 150 ns. Transient photocurrent measurement at (c) current generation stage and (d) current decay stage. TRPL measurements were conducted to observe the quenching ability of each ETL, with a neat perovskite film on bare glass used as a control. The overall decays of perovskite/SnO2@aTiO2/FTO and perovskite/SnO2 /FTO were much faster than that of perovskite/a-TiO2/FTO. This result also suggested that the greater ∆G by direct contact of SnO2 and perovskite is first considered to obtain effective electron extraction (Figure 5a). It is well known that in bi-exponential decay dynamics as seen in the neat perovskite film on bare glass (black squared in Figure 5a), there basically exist two dominant decay components containing the trap-assisted (at the grain boundaries of perovskites) SRH recombination40, 41 and the intrinsic band-to-band radiative recombination mainly before and after around 30 ns. In the case with the ETLs on FTO, the decay constants were derived from fitting process using biexponential function (Table S6). The faster decay constants of perovskite/SnO2@a-TiO2/FTO compared to that of perovskite/SnO2/FTO was clearly seen (Figure 5b), suggesting that the more electrons were extracted rapidly to the FTO through the ETL. This result was consistent with the photovoltaic performances. The transient photocurrent of the solar cells was measured to accurately identify the electron extraction ability of each ETL (see Figure S15 for the full graph). When light illuminated the a-TiO2 device (see Figure 5c), the current density gradually increased and stabilized after 300 µs. On the other hand, the current densities of the SnO2 and


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SnO2@a-TiO2 devices were abruptly increased at the initial stage and stabilized in a shorter time than that of the a-TiO2 device. In comparison of the SnO2 and SnO2@a-TiO2 devices, the current density of the SnO2@a-TiO2 device was stabilized within 100 µs and the stabilized current was over 99% of the initial current, which was slightly better than that of the SnO2 device. After illumination was terminated (Figure 5d), the current density of the SnO2@a-TiO2 device also decreased slightly faster than the SnO2 device. These results clearly demonstrate the effect of physical contact as well as ∆G on electron extraction in the n-i-p type of P-PSCs. In summary, the parameters that can influence the electron extraction in P-PSCs were studied. ∆G, µe, and physical contact were considered, and the higher the free energy difference of the material in contact with the perovskite, the better the electron extraction when the µe values of the ETLs are higher than 10-5 cm2/Vs. In addition, by employing a defect-free a-TiO2 layer, the SnO2@a-TiO2 ETL device showed a 21.1% J–V scan efficiency and a 20.2% MPP efficiency with significantly reduced hysteresis. This study provides insights into design strategies of ETLs and highlights the importance of ∆G and geometrical properties to maximize electron extraction in P-PSCs. ASSOCIATED CONTENT Supporting Information. Full UPS spectra and Magnified UPS spectra, UV-vis absorbsion spectra, SCLC measurements of ETLs, SEM images, HR-TEM images, contact angle measurements, J–V curves of devices, IPCE measurement and transient photocurrent measurements. AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] and [email protected]


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ACKNOWLEDGMENT This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (Code No.2015R1A2A1A10054230). T.P thanks the DONGJIN SEMICHEM Scholarship Foundation for the financial support. S.S and G.K contributed equally to this work.

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(13) Eperon, G. E.; Stranks, S. D.; Menelaou, C.; Johnston, M. B.; Herz, L. M.; Snaith, H. J. Formamidinium lead trihalide: a broadly tunable perovskite for efficient planar heterojunction solar cells. Energy. Environ. Sci. 2014, 7, 982-988 (2014). (14) Xu, X.; Zhang, H.; Shi, J.; Dong, J.; Luo, Y.; Li, D.; Meng, Q. Highly efficient planar perovskite solar cells with a TiO2/ZnO electron transport bilayer. J. Mater. Chem. A 2015, 3, 19288-19293. (15) Chen, B. X.; Rao, H. S.; Li, W. G.; Xu, Y. F.; Chen, H. Y.; Kuang, D. B.; Su, C. Y. Achieving high-performance planar perovskite solar cell with Nb-doped TiO2 compact layer by enhanced electron injection and efficient charge extraction. J. Mater. Chem. A 2016, 4, 56475653. (16) Yang, D.; Yang, R. X.; Zhang, J.; Yang, Z.; Liu, S.; Li, C. High efficiency flexible perovskite solar cells using superior low temperature TiO2. Energy. Environ. Sci. 2015, 8, 32083214. (17) Song, J.; Zheng, E.; Bian, J.; Wang, X. F.; Tian, W.; Sanehira, Y.; Miyasaka, T. Lowtemperature SnO2-based electron selective contact for efficient and stable perovskite solar cells. J. Mater. Chem. A 2015, 3, 10837-10844. (18) Ke, W.; Fang, G.; Liu, Q.; Xiong, L.; Qin, P.; Tao, H.; Wang, J.; Lei, H.; Li, B.; Wan, J. et al. Low-Temperature Solution-Processed Tin Oxide as an Alternative Electron Transporting Layer for Efficient Perovskite Solar Cells. J. Am. Chem. Soc. 2015, 137, 6730-6733.


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(40) Yamada, Y.; Yamada, T.; Shimazaki, A.; Wakamiya, A.; Kanemitsu, Y.

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Systematically Optimized Bilayered Electron Transport Layer for

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