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Enhanced Performance and Stability of Semitransparent Perovskite Solar Cells Using Solution-Processed ThiolFunctionalized Cationic Surfactant as Cathode Buffer Layer Chih-Yu Chang, Yu-Chia Chang, Wen-Kuan Huang, Kuan-Ting Lee, An-Chi Cho, and Chao-Chun Hsu Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.5b03137 • Publication Date (Web): 26 Sep 2015 Downloaded from http://pubs.acs.org on September 28, 2015
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Chemistry of Materials
Enhanced Performance and Stability of Semitransparent Perovskite Solar Cells Using Solution-Processed Thiol-Functionalized Cationic Surfactant as Cathode Buffer Layer Chih-Yu Chang,* Yu-Chia Chang, Wen-Kuan Huang, Kuan-Ting Lee, An-Chi Cho, Chao-Chun Hsu Department of Materials Science and Engineering, Feng Chia University, No. 100 Wenhwa Rd., Seatwen, Taichung, Taiwan 40724, R.O.C. ABSTRACT: We present a facile and effective method to enhance the performance and stability of perovskite solar cells (PSCs) by the incorporation of solution-processed thiol-functionalized cationic surfactant (11-mercaptoundecyl)trimethylammonium bromide (MUTAB) as cathode buffer layer (CBL). Our results indicate that the thiol function groups on MUTAB tend to react with the incident Ag atoms to form covalent Ag-S bonds, while no reaction is observed in the case of methyl-functionalized counterpart dodecyltrimethylammonium bromide (DTAB). Importantly, the presence of Ag-S bonding exerts multi-positive effects on the interface, including decrease of contact resistance between the active layer and Ag electrode, improvement of ambient and thermal stability, and reduction of percolation threshold of ultrathin Ag film. With these desired interfacial properties, the opaque device delivers high power conversion efficiency (PCE) up to 16.5%, which is superior to those of the devices with DTAB (7.9%) and state-of-the-art CBL ZnO nanoparticles (11.0%). The application of MUTAB CBL in semitransparent (ST) solar cells using ultrathin (8-nm) Ag film as transparent top electrode is also demonstrated, and a remarkable PCE of 11.8% with a corresponding average visible transparency (AVT) of 20.8% are achieved, which represents the highest PCE ever reported for ST PSCs with similar AVT. More significantly, the resulting devices possess good ambient stability.
INTRODUCTION Hybrid organic–inorganic halide perovskite solar cells (PSCs) based on methylammonium lead iodide (CH3NH3PbI3) have revealed to be very promising nextgeneration photovoltaic technology due to their potential their potential for cost-effective manufacturing, light weight, and mechanical flexibility.1-7 So far, the development of PSCs with planar heterojunction structure have suppressed a power conversion efficiency (PCE) of 19%,3 thanks to significant progress made in the improvement of processing techniques for perovskite film and optimization of interfacial characteristics between the active layer and electrode.2-9 Ideally, the work-function (WF) of the cathode and anode should be aligned with the energy of the photo-excited quasi-Fermi levels of active layer to ensure ohmic contact for maximizing achievable opencircuit voltage (Voc) and minimizing energy barrier for charge extraction without causing excessive interface recombination.2-6 For planar heterojunction PSCs, the inverted architecture with a configuration of substrate/anode/hole transport layer (HTL)/perovskite/electron transport layer (ETL)/cathode has drawn much attention due to their low manufacturing temperature and potential for use in plastic flexible devices.9-13 A typically inverted PSC comprises glass substrate/indium tin oxide (ITO)/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)/ peorvskite/[6,6]-phenyl-C61-butyric acid methyl ester (PC61BM)/cathode. Currently, low WF metals such as Al (~4.1 eV) or Ca (~2.9 eV) are used as cathode electrodes to ensure efficient electron extraction.10-12 These low-work-
function metals, however, are susceptible to oxidation in ambient air, leading to insufficient device stability. Although the use of more environmental stable high WF metals such as Au (~5.1 eV) or Ag (~4.6 eV) as cathode is highly desirable, the mismatch between the WF of these metals and the lowest unoccupied molecular orbital (LUMO) level of PC61BM (~4.2 eV) often compromises the device performance.14, 15 To address this issue, an effective approach is to incorporate an additional cathode buffer layer (CBL) between PC61BM layer and high WF cathode.14-16 Very recently, solution-processed molecules that bear cationic moieties (e.g. quaternary ammonium) have been proven to be promising CBL materials for PSCs.9,14-16 The excellent solubility of these molecules in polar solvents enables multilayer film deposition through simple solution-based processing without destroying underlying PC61BM layer. In addition, they can generally induce favorable interfacial dipole to lower the WF of the electrode and consequently a better energy level alignment at PC61BM–electrode interface, leading to significantly improved device performance.9,14-16 On the other hand, semitransparent (ST) solar cells have created a new market for the development of novel value-added products (e.g. power generating color window glasses) as they can harvest solar energy to generate electricity without obscuring the view.17-20 Much research in this field has focused on the development of top transparent electrodes while the bottom ones typically employ ITO electrodes.19-21 Recently, ultrathin Ag film (thickness = 10-20 nm) has been studied as a promising material to serve as the top electrode in ST solar cells,19,20,22 thanks to their several unique advantages such as low electrical re-
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sistance (80% at 550 nm), and mechanical flexibility.23 To make ultrathin Ag film as viable transparent electrode, the asgrown film must have good homogeneity and continuity, which is majorly governed by film nucleation and growth kinetics on substrates. However, Ag films growing on dielectric surfaces typically tend to form isolated islands on the surface (Volmer-Weber growth mode) and consequently reveal a rough surface morphology with large grain sizes and low-conducting feature.24,25 The percolation threshold (i.e. the thickness corresponding to which the film morphology goes from island-like to continuous geometry) reported for Ag films is typical between 10-25 nm,26 which in turn greatly compromises their optical transmittance. An effective strategy to improve the quality of ultrathin Ag film is to incorporate an adequate seed layer, such as metal seeds, metal oxides, or organic molecules.23,27,28 These seed layers can provide dense nucleation centres to reduce the percolation thickness of ultrathin Ag film and increase film uniformity at small thicknesses, ensuring continuous Ag film formation (Stranski–Krastanov growth mode) with optimal transmittance and low electrical resistance. Among various seed layer materials, alkanethiol-based molecules have received considerable attention due to its ease of preparation and facile formation of strong Ag-S covalent bond. The presence of such interfacial bonding can also afford low contact resistance (i.e. low series resistance), strong interface adhesion, and good stability in a variety of atmosphere and temperature.23,29-31 Previously, alkanethiolbased self-assembled monolayer (SAM) has been demonstrated to serve as an effective seed layer to deposit highquality ultrathin Ag films.23 Despite these perceived advantages, the applications of alkanethiols in PSCs have never been reported. In this study, we demonstrate firstly that the performance and stability of PSCs can be greatly improved by the incorporation of thiol-functionalized cationic molecule (11-mecaptoundecyl)trimethylammonium bromide (MUTAB; see Figure 1a for the chemical structure) as CBL. A structural analogue of MUTAB, dodecyltrimethylammonium bromide (DTAB; see Figure 1a for the chemical structure) is also used for comparison. The device structure used herein is glass substrate/ITO/PEDOT:PSS/perovskite/ PC61BM/CBL/Ag, as illustrated in Figure 1a. Both DTAB and MUTAB can be deposited from polar solvents (e.g. isopropanol, methanol) to form smooth film surface with room mean square (rms) roughness of ~3 nm (Figure S1), and the ammonium cations on these molecules can induce the formation of favorable interfacial dipoles to turn high WF Ag electrode into efficient low WF electrode. Very interestingly, despite the high structural similarity between DTAB and MUTAB CBLs, their terminal group (i.e. −CH3 and −SH on DTAB and MUTAB, respectively; see Figure 1a) is found to have a considerable effect on the corresponding device performances. Our results indicate that the thiol groups on MUTAB tend to react with Ag atoms to form covalent Ag-S bonds, while no reaction is observed between DTAB
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and Ag. The presence of Ag-S bonds delivers several remarkable features for the use in PSCs, including low contact resistance between the active layer and Ag electrode, superior ambient and thermal stability, and low percolation threshold of ultrathin Ag film. The resulting opaque device using MUTAB CBL delivers a PCE up to 16.5% (average value = 15.5%), greatly outperforming the devices based on DTAB (7.9%) and the state-of-the-art zinc oxide nanopaticles (ZnO NPs; 11.0%) CBLs. In addition, when applying MUTAB CBL to ST devices using ultrathin (8-nm) Ag electrode, an impressive PCE of 11.79% with a corresponding average visible transparency (AVT; calculated between 380 to 750 nm) of 20.8% are demonstrated, which represents the highest PCE ever reported for ST PSCs with similar AVT. More importantly, the devices using MUTAB CBL also possess good ambient stability. This work is an excellent example of employing thiolfunctionalized cationic molecule as CBL to achieve highperformance, ambient-stable, solution-processed PSCs, and this approach is especially valuable for the development of ST devices. First, we investigated the influence of DTAB and MUTAB films on the work function of Ag electrode. The work function of Ag electrode before and after modification was determined by ultraviolet photoelectron spectroscopy (UPS) measurement. As shown in Figure 1b, the work function value of bare Ag electrode was determined to be 4.62 eV, which is consistent with the reported results.32 When Ag electrode was modified with DTAB and MUTAB films, the WF was reduced to 3.58 eV and 3.43 eV, respectively. These low WF contact at the cathode interface causes better energy level matching with PC61BM layer (Figure 1c) and is preferable for efficient electron extraction, as described previously.5-7 This significant decrease in the WF can be attributed to the formation of favorable interfacial dipole created by the strong interaction between the ammonium cations and the electrode surface, as several recent studies have demonstrated that the presence of ammonium cations at the cathode interface can induce interfacial dipoles toward the active layer that can significantly elevate the vacuum level of the cathode for efficient electron extraction.14,15 Considering the hygroscopic nature of tetra-n-alkyl ammonium bromides, the WF evolution of Ag electrode modified with DTAB and MUTAB films as a function of exposure time to ambient condition (30 °C, 65% relative humidity) was also investigated. As shown in Figure 1d, after ~3 days of exposure, an increase in WF (~1 eV) was observed for DTABmodified sample. In contrast, sample with MUTAB layer was found to be fairly stable, as the change in WF was nearly negligible after more than 7 days of exposure. Since the structural difference between DTAB and MUTAB is terminal groups (−CH3 and −SH), X-ray photoelectron spectroscopy (XPS) was performed to gain a deeper understanding of the interaction between Ag surface and the terminal groups. The S2p spectra of Ag layer modified with DTAB and MUTAB films were presented in Figure 1e. For MUTAB-modified sample, the feature peak of S2p at a binding energy of ~162.7 eV was observed, which is in
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Chemistry of Materials
good agreement with those reported for the reactions of alkanethiols with Ag layer and can be attributed to the formation of Ag-S bonds.33,34 However, no peak was observed in the case of DTAB-modified sample, implying no chemical reaction occurs between Ag and DTAB. Based on these results, we suggest that MUTAB film can react with Ag surface to form Ag-S covalent bonds, thus providing superior ambient stability with respect to DTAB film. The device performances with DTAB and MUTAB CBLs were then studied, and a control device without CBL was also fabricated for comparison. The device configuration was glass substrate/ITO/PEDOT:PSS/perovskite/PC61BM/ with and without CBL/Ag. The perovskite film was prepared by two-step sequential solution deposition, and detail characterization of perovskite layer can be found in our previous study.35 The solar-to-electrical PCEs are evaluated by recording the current-voltage (J−V) characteristics under simulated AM 1.5 G conditions (intensity = 100 mWcm−2). The average values and the corresponding standard deviations of photovoltaic parameters including open-circuit voltage (Voc), short-circuit current density (Jsc), fill factor (FF), and PCE were summarized in Table 1. The J-V characteristics of the best performing devices were also depicted in Figure 2a, and the detailed photovoltaic parameters were summarized in Table S1. For the control device without employing CBL (Device A), an average PCE of 2.4% was attained, with a Jsc of 11.95 mA cm-2, an Voc of 0.50 V, and a FF of 39.36% (Table 1 and Figure 2a). The inferior device performance obtained from control device is consistent with previous findings that a large energy barrier exists between the LUMO level of PC61BM (~4.2 eV) and the work function of Ag (~4.6 eV).14,15 Despite the very slight structural difference between DTAB and MUTAB CBLs, it is very intriguing that the resulting device characteristics are very different. For the device employing DTAB as CBL (Device B), an increased PCE of 7.0% was attained, with a Jsc of 15.65 mA cm-2, an Voc of 1.02 V, and a FF of 44.10% (Table 1 and Figure 2a). More encouragingly, a substantial improvement in PCE (by ~6.5 fold) was observed for the device using MUTAB CBL (device C) as a result of the simultaneously increased all the parameters. As shown in Table 1, device C exhibited an average PCE of 15.50%, with a Jsc of 20.66 mA cm-2, an Voc of 1.02 V, and a FF of 73.53%. In particular, the best performing device showed a PCE of 16.50%, with a Jsc of 20.06 mA cm-2, an Voc of 1.03 V, and a FF of 79.80% (Table S1 and Figure 2a). The measured Jsc agreed very well with the Jsc values integrated from incident photonto-current conversion efficiency (IPCE) spectra (Table 1 and Figure 2b), which confirmed the accuracy of our reported PCE values. It should be noted that both DTAB and MUTAB CBLs were optimized at the thickness of approximately 10 nm (Figure S2) to enable efficient electron extraction from the active layer to the Ag electrode without causing high series resistance. To check the reproducibility of the devices, a histogram of device performance obtained from 50 samples of Device C was summarized in Figure 2c. Importantly, more than 90% of the integrated devices delivered PCE above 15% with low standard devia-
tion (Table 1 and Figure 2c), indicating good reproducibility. This could be explained by superior contact between MUTAB CBL and Ag electrode as discussed previously. In addition, given that planar heterojunction PSCs have been shown to be prone to hysteresis,1,2,5,7 J-V characteristic of the device with MUTAB CBL (Device C) measured under different scan directions and various voltage sweep rates was then analyzed. As shown in Figure S3, our devices exhibited photocurrent hysteresis-free J–V characteristics with different scanning directions and/or voltage sweep rates, which can be ascribed to effective passivation of trap states in the perovskite afforded by PC61BM capping layer.10,36 Considering that ZnO NPs film is commonly employed as CBL for high-performance planar heterojunction PSCs,1,5,7,11 the device using ZnO NPs film as CBL (Device D) is also fabricated for comparison. Encouragingly, the optimized MUTAB-based device (Device C) outperformed the ZnO NPs-based device (Device D), as shown in Table 1 and Figure 2a. The superior PCE can be ascribed to better electrical coherence at the organic/organic interface than organic/inorganic interface.37 These promising results clearly indicate the effectiveness of using MUTAB film as the viable CBL in PSCs. Compared to the device with DTAB CBL, higher device performance with MUTAB CBL mainly came from higher FF value (Table 1 and Figure 2a), which could be rationalized in terms of improved series and shunt resistances. The shunt and series resistances of the devices are derived from the inverse slope of the J−V characteristics under dark conditions close to at 0 V and at 1.5 V, respectively (Table S2 and Figure 2d). Compared to the device with DTAB CBL (Device B), the device with MUTAB CBL (Device C) exhibited lower series resistance and higher shunt resistance (i.e. less charge recombination and leakage current), indicating more effective electron-selective contact. This is consistent with previous findings that the formation of Ag-S covalent bonds can improve the charge selectivity of the devices.29,30 These results elucidate that MUTAB CBL provides superior interface modification than DTAB and ZnO NPs CBLs, ensuring efficient electron extraction and preventing undesirable electron-hole recombination between perovskite layer and Ag electrode. The shelf stability of the devices employing DTAB (Device B) and MUTAB CBLs (Device C) were also examined by monitoring the evolution of their PCE as a function of time during storage under ambient conditions (30 °C, ~60% relative humidity). No extra package or encapsulation layer was used for these devices. After being exposed to 26 days, over 75% of the initial PCE could be retained for the device with MUTAB CBL, while the device with DTAB CBL suffered a significant decrease in efficiency (Figure 3a). This might be associated with superior air stability afforded by MUTAB CBL, as discussed previously (Figure 1d). Notably, we found that the device with MUTAB CBL (Device C) remained stable at elevated temperature (65 °C), even for long periods of storage. As shown in Figure 3b, when stored in inert atmosphere at elevated temperature of 65 °C for 6 days, Device C main-
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tained ~95% of initial efficiency. In contrast, a more significant drop in the performance was observed for the device with DTAB CBL (Device B). This result might be related to the robust Ag-S bonds at the MUTAB/Ag interface, as previous findings have shown that the presence of Ag-S bonding can provide good robustness and stability of the interface.23,31 Combining the advantages of good air and thermal stability exhibited in MUTAB CBL-based device, we anticipate that the device stability can also be secured under continuous illumination in ambient air. Another benefit of MUTAB CBL for PSCs was its high applicability in ST devices using ultrathin Ag film as the top electrode. The effects of ultrathin Ag films with nominal thickness between 4 and 16 nm on the PCEs of the devices with DTAB and MUTAB CBLs were depicted in Table 2. The J-V characteristics of the best performing devices were also shown in Figure 4a and 4b, and the detailed photovoltaic parameters were summarized in Table S3. A clear trend in device characteristics with decreasing Ag film thicknesses could be observed: the device with thinner Ag layer delivered lower PCE due to the less light reflection and higher electrical resistance from ST ultrathin Ag electrode. Importantly, all the devices with MUTAB CBL greatly outperformed those with DTAB CBL (Table 2 and Figure 4c). In particular, the performance difference became more pounced especially in the case of Ag films with smaller thicknesses. For the device with 4nm thick Ag film, no photovoltaic response was measurable for the device with DTAB CBL (Device E), while MUTAB-based device (Device I) delivered a moderate PCE of 5.63% (Table 2). Encouragingly, for the devices with 8-nm thick Ag film, a high PCE of 11.04% was attained with MUTAB CBL (Device J), greatly outperforming DTAB CBL-based device (Device F; PCE = 0.71%), as shown in Table 2 and Figure 4c. For the best performing Device J, a remarkable PCE of 11.79% was attained (Table S3 and Figure 4b). It should be emphasized again that all key parameters of the ST device with MUTAB CBL were highly reproducible with low standard deviation (Table 2), presumably due to the good contact between Ag and MUTAB layer. To gain further insight into the improvement afforded by MUTAB layer, the electrical resistances of ultrathin Ag films growth on these layers were then studied. For Ag-coated DTAB layer, a dramatic reduction of the sheet resistance was observed as the Ag thickness reached 16 nm (Figure 4d), suggesting that 16 nm was percolation threshold. While in the case of MUTAB, a much lower percolation threshold of Ag film was observed (8 nm; Figure 4d). The resistance differences of these samples could also be reflected in the morphological analyses. The atomic force microscopy (AFM) images of Ag films growth on DTAB and MUTAB layers were shown in Figure 5. For the samples with DTAB layer, discrete film morphologies were observed regardless of the Ag thickness, with rms surface roughness in the range of 15-36 nm (Figure S4). In contrast, the samples with MUTAB layer revealed relatively smooth (rms roughness in the range of 3-6 nm; see Figure S4) and homogeneous surface morphologies, even with Ag film thickness below
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10 nm. In addition to these desired characteristics, the low percolation threshold of Ag film obtained by employing MUTAB layer was also favorable for light transmittance. For the percolation threshold of Ag films growth on these CBLs (i.e. 16 and 8 nm for DTAB and MUTAB layer, respectively), sample with MUTAB layer showed much higher AVT than that of DTAB layer (49% vs. 33%; Figure S5). These results clearly indicate that MUTAB layer is very promising to promote the nucleation of ultrathin Ag layer and subsequent growth of dense and uniform films with low electrical resistance and high optical transmittance, making it highly suitable for applications in ST devices. The transmittance spectra of MUTAB CBL-based ST devices with different Ag film thicknesses were shown in Figure 6a. As expected, the optical transmittance was increased with reducing Ag film thickness. Notably, for the ST device with 8-nm Ag electrode (Device J), a reasonable AVT of 20.8% was attained (Figure 6a). A photographic image of this device was also shown in Figure 6b, where the campus building and cloudy scene can be visualized clearly through this device. The best PCEs for Device J and K as a function of AVT were depicted in Figure 6c, and the characteristics of state-of-the-art ST PSCs were also included for comparison. Encouragingly, the ST devices demonstrated herein represent the highest PCE compared with other ST PSCs with similar transmittances (Figure 6c). We also noticed that MUTAB CBL-based ST device (Device J) showed good ambient stability (Figure 6d): over 70% of the initial PCE could be retained after being exposed to air (30 °C, ~60% relative humidity) for 14 days without encapsulation. These remarkable results can be ascribed to the fact that MUTAB CBL exerts multiple positive effects on the device characteristics, including superior charge selectivity, low contact resistance, strong interface adhesion, and good capability to promote the uniform nucleation of ultrathin Ag film. CONCLUSION A simple and effective method to improve the performance and stability of PSCs is demonstrated by the incorporation of solution-processed thiol-functionalized cationic surfactant MUTAB as CBL. In contrast to DTAB counterpart, the thiol function groups on MUTAB tend to react with the incident Ag atoms to form covalent Ag-S bonds, which exerts multi-positive effects on the interface. The opaque device with MUTAB CBL exhibited a remarkable PCE up to 16.5% with high reproducibility (average PCE = 15.5% with a standard deviation of 0.4%), greatly outperforming the device with DTAB (7.9%) and the state-of-the-art CBL ZnO NPs (11.0%). In addition, the applicability of using MUTAB CBL in ST solar cells is also demonstrated, and a reasonable AVT of 20.8% with a record high PCE of 11.8% is achieved. More significantly, the resulting devices also possess good ambient stability. The present findings open a new avenue for the integration of a new class of easily-accessible, solution-processed thiol-
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functionalized cationic molecule as CBL to achieve highperformance and ambient-stable perovskite solar cells. EXPERIMENTAL SECTION Materials: Patterned ITO-coated glass substrates with a sheet resistance of 15 ohm sq-1 were purchased from Ruilong Tech. PEDOT:PSS aqueous solution (CLEVIOS P VP Al 4083) was purchased from Heraeus. Methylammonium iodide (CH3NH3I, >99.5%) was purchased from Lumtec. PC61BM (>99.5%) was purchased from Solenne. Unless otherwise stated, all chemicals were purchased from Sigma-Aldrich and used as received. Synthesis of ZnO NPs: ZnO NPs were synthesized by a solution-precipitation process according to literature procedures.4 Briefly, zinc acetate dihydrate (2.95 g) was dissolved in methanol (125 mL) at room temperature. A potassium hydroxide solution (1.48 g in 65 mL methanol) was then added dropwise within 30 min and stirred for 3 hr at 65 °C. The cooled-down solution was then decanted and the precipitate washed twice with ethyl acetate and ethanol. Afterward, ethanol was added to disperse the precipitates and produce ZnO NPs solution. Solar cell fabrication: ITO-coated substrates were cleaned stepwise in detergent, water, acetone, and isopropyl alcohol under ultrasonication for 20 min each and subsequently pretreated by UV-ozone for 60 min. PEDOT:PSS layer (25 nm) was spin-coated on an ITO surface and then annealed at 120 °C for 15 min. The CH3NH3PbI3 perovskite layer (~220 nm) was prepared following two-step solution deposition, as described in our previous work.35 Briefly, lead iodide (PbI2) and methyl ammonium iodide (MAI) were dissolved into dimethylformamide (DMF) and 2propanol with concentrations of 450 mg ml-1 for PbI2 and 40 mg ml-1 for MAI, respectively. Both solutions and substrates were heated at 100 °C for 10 min before being used. The PbI2 solution was spun on preheated substrate (5000 rpm for 40 sec) and then annealed at 70 °C for 10 min. The MAI solution was then spun on top of dried PbI2 film (6000 rpm for 30 sec), followed by annealing at 100 °C for 2 hr. PC61BM layer (60 nm) was then casted by spincoating a solution (15 mg mL-1 PC61BM in chloroform) on top of the formed perovskite layers (1000 rpm for 60 sec). Afterward, CBL (10 nm) was deposited by spin-coating DTAB or MUTAB solution (5 mg mL-1 in isopropanol; 3000 rpm for 60 sec), followed by annealing at 100 °C for 10 min. The opaque Ag (150 nm) or ST Ag layer was then deposited from thermal evaporator under high vacuum (
