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Thin metal top electrode and interface engineering for efficient and air-stable semitransparent perovskite solar cells
Journal of Alloys and Compounds 797 (2019) 65e73
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Thin metal top electrode and interface engineering for efficient and air-stable semitransparent perovskite solar cells Hyun-Jung Lee b, Se-Phin Cho b, Seok-in Na b, Seok-Soon Kim a, * a
Department of Nano & Chemical Engineering, Kunsan National University, 290-2, Miryong-dong, Gunsan-si, Jeollabuk-do, 573-701, Republic of Korea Professional Graduate School of Flexible and Printable Electronics and Polymer Materials Fusion Research Center, Chonbuk National University, 664-14, Deokjin-dong, Deokjin-gu, Jeonju-si, Jeollabuk-do, 561-756, Republic of Korea
b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 7 February 2019 Received in revised form 30 April 2019 Accepted 4 May 2019 Available online 6 May 2019
To find promising device architecture for air-stable as well as efficient semitransparent perovskite solar cells (PeSCs), Ag & Cu semitransparent top electrode and interfacial engineering technologies are explored in PeSCs consisting with indium tin oxide (ITO)/hole transporting layer (HTL)/CH3NH3PbI3/ [6,6]-phenyl-C61 butyric acid methyl ester (PCBM)/Ag or Cu. Devices with NiOx HTL exhibits better performance than conventional poly (3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) based devices and modification of PCBM with polyethylenimine ethoxylated (PEIE) also induces improved performance. Optimized semitransparent PeSC with Cu shows superior performance, achieving a power conversion efficiency (PCE) of 11.95% and average visible transmittance (AVT) of ~20%, and stability to Ag based device. © 2019 Published by Elsevier B.V.
Keywords: Perovskite solar cells Semitransparent NiOx PEIE Ag Cu
1. Introduction Organic-inorganic lead halide PeSCs have attracted tremendous attention owing to their high PCEs more than 20% and great potential for low cost and simple fabrication [1e8]. With the rapid increase of efficiency, recent progress in stability issue, which is one of the most important challenges for practical applications, has indicated their promising future as a result of intensive studies on the degradation mechanism of device, intrinsic instability of the bulk perovskite material, and interface of each component [9e12]. Among various commercialization areas of PeSCs using their outstanding merits, interest in semitransparent power generating applications has emerged [13e15]. Semitransparent solar cell is a multifunctional device combining the ability of electricity generation and the transparency of visible light. Therefore, this technology opens up new opportunity as power generating building and transportation by the integration to windows, skylight, and sunroof. Furthermore, semitransparent PeSCs can be used to enable highly efficient perovskite/conventional inorganic tandem solar cells without significant additional cost due to their solution
* Corresponding author. E-mail address: [email protected] (S.-S. Kim). https://doi.org/10.1016/j.jallcom.2019.05.051 0925-8388/© 2019 Published by Elsevier B.V.
processing capability [16,17]. For versatile applications of semitransparent PeSCs, modulation of transmittance of each component (substrate, photoactive material, interfacial layers, and electrodes) is needed to meet required AVT of full device of ~25% or more. Because organic-inorganic lead halide perovskites have high absorption coefficient, these materials are considered as promising photoactive layers to realize highly transparent cells. The simple way to achieve semitransparent perovskite layer is decrease of perovskite thickness, but there is a trade-off between transparency and overall performance of devices. Hence, optimization of perovskite morphology or development of special structure such as microstructure array of ‘perovskite islands’ as a result of spontaneous de-wetting of perovskite precursor have been reported [18e21]. Discontinuous perovskite layer composed of ‘islands’ absorbing enough sunlight and ‘de-wet regions’ providing visible transparency results in semitransparent PeSCs without significant loss of efficiency. Also, planar architecture is better choice than the porous metal oxide based systems due to minimized light scattering [22]. At the same time as perovskite layer, transparent or semitransparent top and bottom electrodes are required to demonstrate semitransparent PeSCs. Since general device configurations consists of perovskite absorber and charge transporting layers
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deposited on top of the transparent conductive oxide (TCOs) such as ITO and fluorine-doped tin oxide (FTO), the simplest and most effective way to have semitransparent properties in PeSCs is to make transparent top electrode to permit transmission of incident light after passing through absorbing perovskite layer [13]. Various types of top transparent electrodes such as thin metal layer, thin metal layer combined with wide band gap dielectric layer, solution processed Ag nanowire, and carbon nanostructures have been explored for ITO and FTO based semitransparent PeSCs [18,19,23e26]. Among various candidates, a smooth and thin metal layer, which can be easily prepared by thermal evaporation, is compatible with roll-to-roll based mass production and flexible applications [27]. Widely used top electrodes in TCO based PeSCs are aluminum (Al), silver (Ag), and gold (Au). However, fast oxidizing property of Al results in severe degradation of electrical conductivity of electrodes. In addition, Al and Ag induce initial degradation of PeSCs as a result of chemical reaction with perovskite layer [28,29]. Even chemically less active Au is known to react with perovskite at the interface [30]. Here, we reports on ITO based semitransparent PeSCs with thin Ag and Cu top electrode. Although Cu has similar conductivity with Ag, it is significantly abundant and cheap comparing to Ag [31]. Even better, very recently, Jinsong Huang's group has found that Cu can be stable even in direct contact with as a result of accelerated degradation study [32,33]. Nevertheless, there is only few reports related to Cu contained transparent electrodes for semitransparent PeSCs. J. Zhao et al. reported 9.6% semitransparent PeSCs with SnOX/Cu/SnOX top electrode and G. Giuliano et al. have studied on the effect of Au seed layer in the MoOX/Cu/MoOX top electrode system [34,35]. To find promising thin metal top electrode for airstable as well as efficient semitransparent PeSCs, we investigated electrical and optical characteristics of Ag and Cu thin films and their effect on overall performance and air-stability of biplanar PeSCs. Further optimization of devices was systematically carried out by the control of interface of ITO/perovskite and PCBM/thin metal electrode, respectively. Degradation of metal electrodes by the products of perovskite decomposition is more problematic for semitransparent PeSCs based on ultrathin metal films. Hence, we focused on highly resistant Cu semitransparent top electrode and interfacial engineering technologies are explored to find promising device architecture for air-stable as well as efficient semitransparent PeSCs. Because optimized components such as interfacial material is not necessarily transferrable between different device architectures, understanding such discrepancy and optimization in detail can be considered as notable studies for the development of special application such as semitransparent solar cells. 2. Experimental After clean the ITO coated glass substrates (Samsung Corning Co.) with detergent and deionized water (DI) water, sonication in DI water, acetone, and isopropanol was carried out sequentially. Before the preparation of HTLs, substrates were treated with UV/O3 plasma for 20 min to make hydrophilic surface. Conventional PEDOT: PSS and sol-gel derived NiOx was used as HTLs to find the effect of HTLs on performance and stability. Commercialized PEDOT:PSS (Heraeus, CleviosTM P VP AI 4083) was spin-coated by two-step (at 500 rpm for 5 s and 5000 rpm for 40 s) and annealed at 120 C for 10 min in air. For the sol-gel derive NiOx HTLs, 0.1 M nickel acetate (Sigma-Aldrich) mixed in ethanol with 6 vol % ethanolamine was spin-coated at 3000 rpm for 40 s using precursor solution followed by annealing at 350 C for 30 min in air. Then, the perovskite layer was spin-coated at 500 rpm for 5 s and 5000 rpm for 45 s using 35 wt% solution of CH3NH3I and PbI2 with a 1:1 M
ratio in N, N-dimethylmethanamide (DMF) in a glove box filled with N2. During the second step of spin-coating process, 0.7 ml toluene was dropped to obtain high quality perovskite films. Thermal annealing was performed at 100 C for 10 min. The PCBM solution (20 mg PCBM in 1 ml chlorobenzene) was spin-coated at 1000 rpm for 60 s on perovskite films. To modify the interface of PCBM/semitransparent metal layer, 0.5 wt% PEIE in methanol was spin-coated at 5000 rpm for 40 s. Finally, Cu (8 nm) and Ag (8 nm) were thermally evaporated as top metal electrodes, respectively, in vacuum of 10 6 Torr. Optical transparency and electrical conductivity of thin metal top electrodes were characterized by using UVevis spectrophotometer (Varian AU/DMS-100S) and 4-point-probe measurement (FPP-RS9, Dasol Eng.). Valence bands (VBs) of PEDOT:PSS and NiOx HTLs were measured using ultraviolet photoelectron spectroscopy (UPS, ESCALAB 210) with a He I (21.2 eV) source. The photocurrent density-voltage (J-V) curves were obtained using a Keithley 2400 under the condition of 100 mW/cm2 illumination and AM 1.5 G condition after calibration of light intensity with certified reference silicon solar cell. To study performance deviation in a batch, 4 devices were fabricated in each condition and characterized. Statistics analysis was performed by characterizing 28 devices for each condition. Also, change in performance with exposure time in atmospheric condition was recorded without any encapsulation (samples are kept in atmospheric condition, not under the continuous illumination). Field-emission scanning electron microscopy (FE-SEM, HITACHI-SU8220) was used to observe device structure after stability test. 3. Results and discussion As mentioned, the simplest way to demonstrate semitransparent PeSCs is replacement of opaque metal top electrode to transparent electrode in general TCO based device configurations. Among various candidates as top transparent electrodes, we investigated ultrathin Ag can Cu electrodes as shown in Fig. 1a. Because the main purpose of this study is not only to achieve high efficiency but also to obtain excellent long-term device stability, Cu electrode, which is known to highly resistant to the formation of halide by the chemical reaction with perovskite, was carefully studied and compared with general Ag electrode. Interface of ITO/ perovskite and PCBM/thin metal electrode was also systematically studied to further optimize. PEDOT: PSS, which is commonly used HTL in conventional polymer solar cells due to its high transparency, high work function, and simple processability, has also been used as typical HTL in PeSCs. However, owing to its highly acidic and hygroscopic properties causing poor device stability, PEDOT:PSS alternatives such as carbon based nanostructure, polymer composite, and metal oxide have been studied by several groups [36e41]. Moreover, large mismatch between VB of PEDOT:PSS of ~5 eV and deep lying VB of perovskite such as CH3NH3PbI3 of ~5.4 eV limits sufficient carrier separation and collection to each electrode. Among various candidates, improvement of PCE and stability have been demonstrated by using NiOx HTL as a result of improved wetting of CH3NH3PbI3 on NiOx and better aligned energy level between NiOX and CH3NH3PbI3 [40,41]. Hence, we applied PEDOT:PSS and NiOx as HTLs and their effect on the performance and stability of semitransparent PeSCs was studied. As shown in the energy band diagram for each layer in Fig. 1b, the VB of our NiOx was evaluated to ~5.25 eV by UPS measurement and which is higher than that of PEDOT:PSS (4.95e5.00 eV). It indicates that closer VB of NiOx to VB of CH3NH3PbI3 (5.4 eV) induced higher open-circuit voltage (Voc) than that of PEDOT:PSS based devices. Well-matched VBs between NiOx and CH3NH3PbI3 result in more efficient hole collection at the ITO too.
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Fig. 1. (a) Schematic layout of the semitransparent PeSCs. (b) Energy band diagram.
Because very thin metal layer acts as top electrode to obtain suitable transparency, electrical property of top electrode is poor comparing to opaque device and thus may be unfavorable for charge extraction & collection to top electrode. Hence, control of interface between PCBM and metal top electrode is also required to minimize the contact barrier limiting sufficient electron extraction. Thermally evaporated LiF and bathocuproine (BCP), metal oxide such as Nb doped TiOx and ZnO, and polyelectrolyte have been used to increase device efficiency by improving contact properties [42e45]. Among various interface modifier, we used PEIE, which can be simply prepared by spin-coating in the ambient atmosphere. As shown in Fig. 1b, after modification by PEIE, the work function of Ag and Cu was decreased to 3.9 and 4.0 eV, which are originally 4.6 eV or 4.7 eV. It is attributed to that the neutral amine groups in the PEIE produce dipole moments at the interface and induce significant change in the workfunction of Ag and Cu. Consequently, the contact barrier between the metal electrode and the PCBM can be improved and efficient electron extraction to the top electrode is allowed. Although excellent optical transparency and electrical conductivity are two important requirements for top electrodes to demonstrate efficient semitransparent, there is always a trade-off. Hence, optimal thickness, which has sufficient electrical conductivity and maximized transmittance, is needed and 8 nm Ag and Cu was chosen as a result of basic study. The electrical properties of electrodes are shown in Fig. 2a. Comparing to ITO front transparent electrode having a sheet resistance (Rsheet) of ~13 U/cm2, Cu and Ag thin electrodes showed lower Rsheet of 5.37 U/cm2 and 9.86 U/cm2, respectively. However, both Ag and Cu thin electrodes showed poor transmittance in the whole wavelength regions and this properties must be improved for the application to tandem cells to maximize
light harvesting in the upper cells. As shown in photographs in Fig. 2b, semitransparent PeSCs fabricated with Ag and Cu electrode exhibit brown color due to intense absorption of light below 700 nm by perovskite absorber. The AVT was evaluated as the average of the transmittance in the visible region. AVT of the device without top electrode was estimated to ~33% and which value is similar with previous reports that has a ~150 nm thick perovskite layer [18] Full devices with Ag and Cu electrode exhibited decreased AVT of ~20%. Considering minimum AVT requirement of 20e25% for real window application, semitransparent PeSCs investigated in this study is suitable for various application areas. To verify possibility of thin Ag and Cu as top electrode in semitransparent solar cells, we analyzed J-V characteristics. As described, there is a trade-off between AVT and photovoltaic performance in semitransparent solar cells. Although AVT of devices meets requirement for real application, realization of these devices must be limited without sufficient J-V performance. Fig. 3 shows important parameters in J-V performances of devices with Ag and Cu top electrodes, respectively. Deviations in PCE, fill factor (FF), Voc, and short-circuit current density (Jsc) of 4 devices fabricated in a batch per each condition were recorded. As shown in Fig. 3a and b, NiOx HTL based PeSCs showed higher efficiency than conventional PEDOT:PSS based devices in both Ag and Cu systems. In particular, Voc was considerably increased. As described, closer VB of NiOx to VB of CH3NH3PbI3 induced higher Voc and more efficient hole collection at the ITO. Furthermore, because NiOx has small electron affinity, effective blocking of electrons can minimize recombination with the holes at the ITO. Hence, NiOx based PeSCs exhibited superior performance to conventional PEDOT:PSS based devices. Next, when the interface of PCBM and metal electrode was modified with PEIE, device performances were better than those without
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Fig. 2. (a) Transmittance spectra of electrodes and device with and without top electrodes. (b) Rsheet of ITO, Ag, and Cu on glass substrate.
PEIE. Dipole moments at the interface produced by the neutral amine groups in the PEIE results in improved contact barrier between the metal electrode and the PCBM and thus efficient electron collection to the top electrode is allowed, leading to higher device performance. For further confirmation on the best conditions, effect of top metal electrode was investigated. Cu and Ag based opaque and semitransparent PeSCs were fabricated with optimized interface and their performance was compared. As shown in representative J-V curves in Fig. 4a and summarized parameters in Table 1, both opaque and semitransparent devices with Cu top electrode showed slightly higher performance than that with Ag top electrode. In case of opaque device with Cu showed highest performance, achieving a Voc of 1.02 V, Jsc of 17.98 mA/cm 2, FF of 77.80%, and PCE of 14.29%. Here, PCE of ~14% is reasonable values considering relatively thin perovskite layer of ~150 nm. To demonstrate proper performance as well as transparency, 150 nm perovskite layer with smooth surface
is selected as optimal condition for our semitransparent PeSCs. Cu based semitransparent PeSC showed Voc of 1.01 V, Jsc of 16.19 mA/ cm 2, FF of 73.05%, and PCE of 11.95%. The performance and optical transparency reported to date is vary depending on the device structure and materials. For better understanding, we summarized recent reports on semitransparent PeSCs in Table S1 in supporting information. As you can see in Table S1, PeSCs achieving AVT of ~20% exhibited PCE of ~10% or lower. Therefore, we conclude that our semitransparent PeSCs are comparable to the best performance reported to date. Comparing to opaque device, semitransparent PeSC exhibited lower FF and Jsc. A series resistance (Rs) of opaque and semitransparent PeSCs were calculated to 2.36 U cm2 and 7.35 U cm2, respectively. Although slightly lower FF was attributed to higher (Rs) in thin Cu electrode system, still high FF of ~73.05% indicates that our thin Cu is enough to restrict loss of photogenerated carriers during collection and lateral transport steps. Thick Cu in the opaque PeSC serves as a rear reflector, increasing the
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Fig. 3. Photovoltaic parameters for semitransparent PeSCs with controlled interface; (a) Cu top electrode system. (b) Ag top electrode system.
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Fig. 4. (a) Representative J-V characteristic of the optimized opaque & semitransparent PeSCs. (b) Histograms of PCE for optimized semitransparent PeSCs.
Table 1 Photovoltaic characteristics of optimized opaque & semitransparent PeSCs.
Cu Ag Cu Ag
(40 nm) e Opaque PeSC (40 nm) e Opaque PeSC (8 nm) e Semi-transparent PeSC (8 nm) e Semi-transparent PeSC
Voc (V)
Jsc (mA/cm2)
Fill Factor
Efficiency
1.02 1.07 1.01 1.06
17.98 16.51 16.19 14.56
77.81 77.33 73.05 71.78
14.29 13.66 11.95 11.05
path length of incident light in the perovskite layer, and thus more harvesting of photons is possible. In contrast, the transmittance of 8 nm Cu leads to Jsc loss owing to nonaborbed photons passing through semitransparent Cu. Semitransparent PeSC with Ag also showed slightly lower performance comparing to opaque device due to same reasons with Cu based devices. As mentioned, Cu based devices showed slightly higher performance than Ag based system. Although we performed various characterization to verify the reason, we can not find any difference. But, as shown in the PCE statics (Fig. 4b) of devices fabricated in 4 different batches, Cu based semitransparent PeSCs showed higher efficiencies and narrower deviations than that of Ag based system. One of possible reason is better contact of Cu based device than that of Ag based system. Because Cu has a higher surface energy than Ag, interface between
PEIE modified PCBM and thin Cu is superior to that of Ag [46] Thus, better electron collection to Cu top electrode can be expected. Because reproducibility of device performance is critical issues in the commercialization stage to determine yield and overall production cost, minimizing the deviation in PCE and stability is important. Considering this point, Cu is better choice than Ag in thin metal based semitransparent PeSCs. As mentioned, the improvement of stability is equally important purpose to high efficiency in this study. Therefore, we monitored the change of PCE with exposure time in an ambient conditions (temperature of ~22 C and humidity of 40e50%) without any encapsulation and showed in Fig. 5a. Here, 3 devices per each configuration were tested in the same conditions. In case of Cu based system, PEDOT:PSS devices showed a gradual decline in PCE
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Fig. 5. (a) Change in PCEs during exposure to air without any encapsulation. (b) Cross sectional images of devices after stability test.
with time, while PCE of devices with NiOx was retained for 6 days without notable change. Replacement of hygroscopic and acidic PEDOT:PSS, which easily absorbs water from the environment and induces etch of the transparent electrode, with NiOX results in improved stability. Although very thin Cu of 8 nm was used as top electrode, this type of devices show excellent stability (80% PCE after 20 days). In contrast, some devices with Ag showed sharp decrease in PCE even though NiOx was used as HTL. After stability test, the cross-sectional SEM image of devices were characterized. Clear interface between each layer can be observed in the Cu based stable devices, while layered heterojunction of perovskite and PCBM was destructed in the case of Ag based device exhibiting poor stability.
4. Conclusion To demonstrate semitransparent PeSCs, opaque metal top electrode is replaced to semitransparent Ag & Cu electrode in
general ITO based device configurations. Although Cu has similar electrical conductivity with Ag, it is cheap and known to chemically stable even in direct contact with perovskite. Not only to achieve high efficiency but also to obtain excellent long-term device stability, Cu was carefully studied and compared with general Ag electrode. Further optimization of devices was systematically carried out by the control of interface of ITO/perovskite and PCBM/thin metal electrode, respectively. NiOx HTL based PeSCs showed higher efficiencies than conventional PEDOT:PSS based devices in both Ag and Cu systems due to more efficient hole collection resulted from well-matched energy level with VB of perovskite and effective blocking of electrons. When the PCBM was modified with PEIE, device performances were better than those without PEIE owing to improved contact barrier between the metal electrode and the PCBM. As a result, excellent Cu based semitransparent device with AVT of ~20%, achieving a Voc of 1.01 V, Jsc of 16.19 mA/cm 2, FF of 73.05%, and PCE of 11.95%, was successfully demonstrated. This type of device (using NiOx HTL instead of PEDOT:PSS and PCBM modified
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with PEIE) also showed excellent stability (80% PCE after 20 days). In contrast, some devices with Ag showed sharp decrease in PCE even though NiOx was used as HTL. Clear interface between each layer can be observed in the Cu based stable devices, while layered heterojunction of perovskite and PCBM was destructed in the case of Ag based device exhibiting poor stability. These results indicate that NiOx HTL and Cu top electrode is better choice to demonstrate air-stable as well as efficient semitransparent PeSCs. Acknowledgments This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (2018R1A2B6001206). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.jallcom.2019.05.051. References rez-Santiesteban, X. Mathew, [1] S.-H. Turren-Cruz, M. Saliba, M.T. Mayer, H. Jua L. Nienhaus, W. Tress, M.P. Erodici, M.-J. Sher, M.G. Bawendi, M. Gratzel, A. Abate, A. Hagfeldt, J.-P. Correa-Baena, Enhanced charge carrier mobility and lifetime suppress hysteresis and improve efficiency in planar perovskite solar cells, Energy Environ. Sci. 11 (2018) 78e86. [2] D.-Y. Son, S.-G. Kim, J.-Y. Seo, S.-H. Lee, H.J. Shin, D.H. Lee, N.-G. Park, Universal approach toward hysteresis-free perovskite solar cell via defect engineering, Journal of the American Chemical Society, J. Am. Chem. Soc. 140 (2018) 1358e1364. [3] W.S. Yang, B.W. Park, E.H. Jung, N.J. Jeon, W.C. Kim, D.U. Lee, S.S. Shin, J.W. Seo, E.K. Kim, J.H. Noh, S.I. Seok, Iodide management in formamidinium-leadhalideebased perovskite layers for efficient solar cells, Science 356 (2017) 1376e1379. [4] H.J. Snaith, Present status and future prospects of perovskite photovoltaics, Nat. Mater. 17 (2018) 372e376. [5] Best Research-Cell Efficiencies. https://www.nrel.gov/pv/assets/images/ efficiency-chart-20180716.jpg. [6] X. Zeng, T. Zhou, C. Leng, Z. Zang, M. Wang, W. Hu, X. Tang, S. Lu, L. Fang, M. Zhou, Performance improvement of perovskite solar cells by employing a CdSe quantum dot/PCBM composite as an electron transport layer, J. Mater. Chem. A. 5 (2017) 17499e17505. [7] M. Wang, Z. Zang, B. Yang, X. Hu, K. Sun, L. Sun, Performance improvement of perovskite solar cells through enhanced hole extraction: the role of iodide concentration gradient, Sol. Energy Mater. Sol. Cells 185 (2018) 117e123. [8] X. Zhao, S. Liu, H. Zhang, S.-Y. Chang, W. Huang, B. Zhu, Y. Shen, C. Chen, D. Wang, Y. Yang, M. Wang, 20% efficient perovskite solar cells with 2D electron transporting layer, Adv. Funct. Mater. 29 (2019) 1805168. n-Carmona, I. Zimmermann, E. Mosconi, X. Lee, [9] G. Grancini, C. Rolda D. Martineau, S. Narbey, F. Oswald, F. De Angelis, M. Graetze, Mohammad Khaja Nazeeruddin, One-Year stable perovskite solar cells by 2D/3D interface engineering, Nat. Commun. 8 (2017) 15684. [10] L. Meng, J. You, Y. Yang, Addressing the stability issue of perovskite solar cells for commercial applications, Nat. Commun. 9 (2018) 5265. [11] X. Qin, Z. Zhao, Y. Wang, J. Wu, Q. Jiang, J. You, Recent progress in stability of perovskite solar cells, J. Semicond. 38 (2017), 011002. [12] T. Zhou, M. Wang, Z. Zang, X. Tang, L. Fang, Two-dimensional lead-free hybrid halide perovskite using superatom anions with tunable electronic properties, Sol. Energy Mater. Sol. Cells 191 (2019) 33e38. [13] K.-T. Lee, L.J. Guo, H.J. Park, Neutral- and multi-colored semitransparent perovskite solar cells, Molecules 21 (2016) 475e495. [14] J. Sun, J.J. Jasieniak, Semi-transparent solar cells, J. Phys. D Appl. Phys. 50 (1e28) (2017), 093001. [15] Q. Xue, R. Xia, C.J. Brabec, H.-L. Yip, Recent advances in semi-transparent polymer and perovskite solar cells for power generating window applications, Energy Environ. Sci. 11 (2018) 1688e1709. [16] D.P. McMeekin, G. Sadoughi, W. Rehman, G.E. Eperon, M. Saliva, M.T. Horantner, A. Haghighirad, N. Sakai, L. Korte, B. Rech, M.B. Johnston, L.M. Herz, H.J. Snaith, A mixed-cation lead mixed-halide perovskite absorber for tandem solar cells, Science 351 (2016) 151e155. [17] B. Chen, Y. Bai, Z. Yu, T. Li, X. Zheng, Q. Dong, L. Shen, M. Boccard, A. Gruverman, Z. Holman, J. Huang, Efficient semitransparent perovskite solar cells for 23.0%-efficiency perovskite/silicon four-terminal tandem cells, Adv. Energy Mater. 6 (2016) 1601128 1e7. [18] C. Roldan-Carmona, O. Malinkiewicz, R. Betancur, G. Longo, C. Momblona, F. Jaramillo, L. Camacho, H.J. Bolink, High efficiency single-junction semitransparent perovskite solar cells, Energy Environ. Sci. 7 (2014) 2968e2973.
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Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Thin metal top electrode and interface engineering for efficient and air-stable semitransparent perovskite solar cells Hyun-Jung Lee b, Se-Phin Cho b, Seok-in Na b, Seok-Soon Kim a, * a
Department of Nano & Chemical Engineering, Kunsan National University, 290-2, Miryong-dong, Gunsan-si, Jeollabuk-do, 573-701, Republic of Korea Professional Graduate School of Flexible and Printable Electronics and Polymer Materials Fusion Research Center, Chonbuk National University, 664-14, Deokjin-dong, Deokjin-gu, Jeonju-si, Jeollabuk-do, 561-756, Republic of Korea
b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 7 February 2019 Received in revised form 30 April 2019 Accepted 4 May 2019 Available online 6 May 2019
To find promising device architecture for air-stable as well as efficient semitransparent perovskite solar cells (PeSCs), Ag & Cu semitransparent top electrode and interfacial engineering technologies are explored in PeSCs consisting with indium tin oxide (ITO)/hole transporting layer (HTL)/CH3NH3PbI3/ [6,6]-phenyl-C61 butyric acid methyl ester (PCBM)/Ag or Cu. Devices with NiOx HTL exhibits better performance than conventional poly (3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) based devices and modification of PCBM with polyethylenimine ethoxylated (PEIE) also induces improved performance. Optimized semitransparent PeSC with Cu shows superior performance, achieving a power conversion efficiency (PCE) of 11.95% and average visible transmittance (AVT) of ~20%, and stability to Ag based device. © 2019 Published by Elsevier B.V.
Keywords: Perovskite solar cells Semitransparent NiOx PEIE Ag Cu
1. Introduction Organic-inorganic lead halide PeSCs have attracted tremendous attention owing to their high PCEs more than 20% and great potential for low cost and simple fabrication [1e8]. With the rapid increase of efficiency, recent progress in stability issue, which is one of the most important challenges for practical applications, has indicated their promising future as a result of intensive studies on the degradation mechanism of device, intrinsic instability of the bulk perovskite material, and interface of each component [9e12]. Among various commercialization areas of PeSCs using their outstanding merits, interest in semitransparent power generating applications has emerged [13e15]. Semitransparent solar cell is a multifunctional device combining the ability of electricity generation and the transparency of visible light. Therefore, this technology opens up new opportunity as power generating building and transportation by the integration to windows, skylight, and sunroof. Furthermore, semitransparent PeSCs can be used to enable highly efficient perovskite/conventional inorganic tandem solar cells without significant additional cost due to their solution
* Corresponding author. E-mail address: [email protected] (S.-S. Kim). https://doi.org/10.1016/j.jallcom.2019.05.051 0925-8388/© 2019 Published by Elsevier B.V.
processing capability [16,17]. For versatile applications of semitransparent PeSCs, modulation of transmittance of each component (substrate, photoactive material, interfacial layers, and electrodes) is needed to meet required AVT of full device of ~25% or more. Because organic-inorganic lead halide perovskites have high absorption coefficient, these materials are considered as promising photoactive layers to realize highly transparent cells. The simple way to achieve semitransparent perovskite layer is decrease of perovskite thickness, but there is a trade-off between transparency and overall performance of devices. Hence, optimization of perovskite morphology or development of special structure such as microstructure array of ‘perovskite islands’ as a result of spontaneous de-wetting of perovskite precursor have been reported [18e21]. Discontinuous perovskite layer composed of ‘islands’ absorbing enough sunlight and ‘de-wet regions’ providing visible transparency results in semitransparent PeSCs without significant loss of efficiency. Also, planar architecture is better choice than the porous metal oxide based systems due to minimized light scattering [22]. At the same time as perovskite layer, transparent or semitransparent top and bottom electrodes are required to demonstrate semitransparent PeSCs. Since general device configurations consists of perovskite absorber and charge transporting layers
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deposited on top of the transparent conductive oxide (TCOs) such as ITO and fluorine-doped tin oxide (FTO), the simplest and most effective way to have semitransparent properties in PeSCs is to make transparent top electrode to permit transmission of incident light after passing through absorbing perovskite layer [13]. Various types of top transparent electrodes such as thin metal layer, thin metal layer combined with wide band gap dielectric layer, solution processed Ag nanowire, and carbon nanostructures have been explored for ITO and FTO based semitransparent PeSCs [18,19,23e26]. Among various candidates, a smooth and thin metal layer, which can be easily prepared by thermal evaporation, is compatible with roll-to-roll based mass production and flexible applications [27]. Widely used top electrodes in TCO based PeSCs are aluminum (Al), silver (Ag), and gold (Au). However, fast oxidizing property of Al results in severe degradation of electrical conductivity of electrodes. In addition, Al and Ag induce initial degradation of PeSCs as a result of chemical reaction with perovskite layer [28,29]. Even chemically less active Au is known to react with perovskite at the interface [30]. Here, we reports on ITO based semitransparent PeSCs with thin Ag and Cu top electrode. Although Cu has similar conductivity with Ag, it is significantly abundant and cheap comparing to Ag [31]. Even better, very recently, Jinsong Huang's group has found that Cu can be stable even in direct contact with as a result of accelerated degradation study [32,33]. Nevertheless, there is only few reports related to Cu contained transparent electrodes for semitransparent PeSCs. J. Zhao et al. reported 9.6% semitransparent PeSCs with SnOX/Cu/SnOX top electrode and G. Giuliano et al. have studied on the effect of Au seed layer in the MoOX/Cu/MoOX top electrode system [34,35]. To find promising thin metal top electrode for airstable as well as efficient semitransparent PeSCs, we investigated electrical and optical characteristics of Ag and Cu thin films and their effect on overall performance and air-stability of biplanar PeSCs. Further optimization of devices was systematically carried out by the control of interface of ITO/perovskite and PCBM/thin metal electrode, respectively. Degradation of metal electrodes by the products of perovskite decomposition is more problematic for semitransparent PeSCs based on ultrathin metal films. Hence, we focused on highly resistant Cu semitransparent top electrode and interfacial engineering technologies are explored to find promising device architecture for air-stable as well as efficient semitransparent PeSCs. Because optimized components such as interfacial material is not necessarily transferrable between different device architectures, understanding such discrepancy and optimization in detail can be considered as notable studies for the development of special application such as semitransparent solar cells. 2. Experimental After clean the ITO coated glass substrates (Samsung Corning Co.) with detergent and deionized water (DI) water, sonication in DI water, acetone, and isopropanol was carried out sequentially. Before the preparation of HTLs, substrates were treated with UV/O3 plasma for 20 min to make hydrophilic surface. Conventional PEDOT: PSS and sol-gel derived NiOx was used as HTLs to find the effect of HTLs on performance and stability. Commercialized PEDOT:PSS (Heraeus, CleviosTM P VP AI 4083) was spin-coated by two-step (at 500 rpm for 5 s and 5000 rpm for 40 s) and annealed at 120 C for 10 min in air. For the sol-gel derive NiOx HTLs, 0.1 M nickel acetate (Sigma-Aldrich) mixed in ethanol with 6 vol % ethanolamine was spin-coated at 3000 rpm for 40 s using precursor solution followed by annealing at 350 C for 30 min in air. Then, the perovskite layer was spin-coated at 500 rpm for 5 s and 5000 rpm for 45 s using 35 wt% solution of CH3NH3I and PbI2 with a 1:1 M
ratio in N, N-dimethylmethanamide (DMF) in a glove box filled with N2. During the second step of spin-coating process, 0.7 ml toluene was dropped to obtain high quality perovskite films. Thermal annealing was performed at 100 C for 10 min. The PCBM solution (20 mg PCBM in 1 ml chlorobenzene) was spin-coated at 1000 rpm for 60 s on perovskite films. To modify the interface of PCBM/semitransparent metal layer, 0.5 wt% PEIE in methanol was spin-coated at 5000 rpm for 40 s. Finally, Cu (8 nm) and Ag (8 nm) were thermally evaporated as top metal electrodes, respectively, in vacuum of 10 6 Torr. Optical transparency and electrical conductivity of thin metal top electrodes were characterized by using UVevis spectrophotometer (Varian AU/DMS-100S) and 4-point-probe measurement (FPP-RS9, Dasol Eng.). Valence bands (VBs) of PEDOT:PSS and NiOx HTLs were measured using ultraviolet photoelectron spectroscopy (UPS, ESCALAB 210) with a He I (21.2 eV) source. The photocurrent density-voltage (J-V) curves were obtained using a Keithley 2400 under the condition of 100 mW/cm2 illumination and AM 1.5 G condition after calibration of light intensity with certified reference silicon solar cell. To study performance deviation in a batch, 4 devices were fabricated in each condition and characterized. Statistics analysis was performed by characterizing 28 devices for each condition. Also, change in performance with exposure time in atmospheric condition was recorded without any encapsulation (samples are kept in atmospheric condition, not under the continuous illumination). Field-emission scanning electron microscopy (FE-SEM, HITACHI-SU8220) was used to observe device structure after stability test. 3. Results and discussion As mentioned, the simplest way to demonstrate semitransparent PeSCs is replacement of opaque metal top electrode to transparent electrode in general TCO based device configurations. Among various candidates as top transparent electrodes, we investigated ultrathin Ag can Cu electrodes as shown in Fig. 1a. Because the main purpose of this study is not only to achieve high efficiency but also to obtain excellent long-term device stability, Cu electrode, which is known to highly resistant to the formation of halide by the chemical reaction with perovskite, was carefully studied and compared with general Ag electrode. Interface of ITO/ perovskite and PCBM/thin metal electrode was also systematically studied to further optimize. PEDOT: PSS, which is commonly used HTL in conventional polymer solar cells due to its high transparency, high work function, and simple processability, has also been used as typical HTL in PeSCs. However, owing to its highly acidic and hygroscopic properties causing poor device stability, PEDOT:PSS alternatives such as carbon based nanostructure, polymer composite, and metal oxide have been studied by several groups [36e41]. Moreover, large mismatch between VB of PEDOT:PSS of ~5 eV and deep lying VB of perovskite such as CH3NH3PbI3 of ~5.4 eV limits sufficient carrier separation and collection to each electrode. Among various candidates, improvement of PCE and stability have been demonstrated by using NiOx HTL as a result of improved wetting of CH3NH3PbI3 on NiOx and better aligned energy level between NiOX and CH3NH3PbI3 [40,41]. Hence, we applied PEDOT:PSS and NiOx as HTLs and their effect on the performance and stability of semitransparent PeSCs was studied. As shown in the energy band diagram for each layer in Fig. 1b, the VB of our NiOx was evaluated to ~5.25 eV by UPS measurement and which is higher than that of PEDOT:PSS (4.95e5.00 eV). It indicates that closer VB of NiOx to VB of CH3NH3PbI3 (5.4 eV) induced higher open-circuit voltage (Voc) than that of PEDOT:PSS based devices. Well-matched VBs between NiOx and CH3NH3PbI3 result in more efficient hole collection at the ITO too.
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Fig. 1. (a) Schematic layout of the semitransparent PeSCs. (b) Energy band diagram.
Because very thin metal layer acts as top electrode to obtain suitable transparency, electrical property of top electrode is poor comparing to opaque device and thus may be unfavorable for charge extraction & collection to top electrode. Hence, control of interface between PCBM and metal top electrode is also required to minimize the contact barrier limiting sufficient electron extraction. Thermally evaporated LiF and bathocuproine (BCP), metal oxide such as Nb doped TiOx and ZnO, and polyelectrolyte have been used to increase device efficiency by improving contact properties [42e45]. Among various interface modifier, we used PEIE, which can be simply prepared by spin-coating in the ambient atmosphere. As shown in Fig. 1b, after modification by PEIE, the work function of Ag and Cu was decreased to 3.9 and 4.0 eV, which are originally 4.6 eV or 4.7 eV. It is attributed to that the neutral amine groups in the PEIE produce dipole moments at the interface and induce significant change in the workfunction of Ag and Cu. Consequently, the contact barrier between the metal electrode and the PCBM can be improved and efficient electron extraction to the top electrode is allowed. Although excellent optical transparency and electrical conductivity are two important requirements for top electrodes to demonstrate efficient semitransparent, there is always a trade-off. Hence, optimal thickness, which has sufficient electrical conductivity and maximized transmittance, is needed and 8 nm Ag and Cu was chosen as a result of basic study. The electrical properties of electrodes are shown in Fig. 2a. Comparing to ITO front transparent electrode having a sheet resistance (Rsheet) of ~13 U/cm2, Cu and Ag thin electrodes showed lower Rsheet of 5.37 U/cm2 and 9.86 U/cm2, respectively. However, both Ag and Cu thin electrodes showed poor transmittance in the whole wavelength regions and this properties must be improved for the application to tandem cells to maximize
light harvesting in the upper cells. As shown in photographs in Fig. 2b, semitransparent PeSCs fabricated with Ag and Cu electrode exhibit brown color due to intense absorption of light below 700 nm by perovskite absorber. The AVT was evaluated as the average of the transmittance in the visible region. AVT of the device without top electrode was estimated to ~33% and which value is similar with previous reports that has a ~150 nm thick perovskite layer [18] Full devices with Ag and Cu electrode exhibited decreased AVT of ~20%. Considering minimum AVT requirement of 20e25% for real window application, semitransparent PeSCs investigated in this study is suitable for various application areas. To verify possibility of thin Ag and Cu as top electrode in semitransparent solar cells, we analyzed J-V characteristics. As described, there is a trade-off between AVT and photovoltaic performance in semitransparent solar cells. Although AVT of devices meets requirement for real application, realization of these devices must be limited without sufficient J-V performance. Fig. 3 shows important parameters in J-V performances of devices with Ag and Cu top electrodes, respectively. Deviations in PCE, fill factor (FF), Voc, and short-circuit current density (Jsc) of 4 devices fabricated in a batch per each condition were recorded. As shown in Fig. 3a and b, NiOx HTL based PeSCs showed higher efficiency than conventional PEDOT:PSS based devices in both Ag and Cu systems. In particular, Voc was considerably increased. As described, closer VB of NiOx to VB of CH3NH3PbI3 induced higher Voc and more efficient hole collection at the ITO. Furthermore, because NiOx has small electron affinity, effective blocking of electrons can minimize recombination with the holes at the ITO. Hence, NiOx based PeSCs exhibited superior performance to conventional PEDOT:PSS based devices. Next, when the interface of PCBM and metal electrode was modified with PEIE, device performances were better than those without
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Fig. 2. (a) Transmittance spectra of electrodes and device with and without top electrodes. (b) Rsheet of ITO, Ag, and Cu on glass substrate.
PEIE. Dipole moments at the interface produced by the neutral amine groups in the PEIE results in improved contact barrier between the metal electrode and the PCBM and thus efficient electron collection to the top electrode is allowed, leading to higher device performance. For further confirmation on the best conditions, effect of top metal electrode was investigated. Cu and Ag based opaque and semitransparent PeSCs were fabricated with optimized interface and their performance was compared. As shown in representative J-V curves in Fig. 4a and summarized parameters in Table 1, both opaque and semitransparent devices with Cu top electrode showed slightly higher performance than that with Ag top electrode. In case of opaque device with Cu showed highest performance, achieving a Voc of 1.02 V, Jsc of 17.98 mA/cm 2, FF of 77.80%, and PCE of 14.29%. Here, PCE of ~14% is reasonable values considering relatively thin perovskite layer of ~150 nm. To demonstrate proper performance as well as transparency, 150 nm perovskite layer with smooth surface
is selected as optimal condition for our semitransparent PeSCs. Cu based semitransparent PeSC showed Voc of 1.01 V, Jsc of 16.19 mA/ cm 2, FF of 73.05%, and PCE of 11.95%. The performance and optical transparency reported to date is vary depending on the device structure and materials. For better understanding, we summarized recent reports on semitransparent PeSCs in Table S1 in supporting information. As you can see in Table S1, PeSCs achieving AVT of ~20% exhibited PCE of ~10% or lower. Therefore, we conclude that our semitransparent PeSCs are comparable to the best performance reported to date. Comparing to opaque device, semitransparent PeSC exhibited lower FF and Jsc. A series resistance (Rs) of opaque and semitransparent PeSCs were calculated to 2.36 U cm2 and 7.35 U cm2, respectively. Although slightly lower FF was attributed to higher (Rs) in thin Cu electrode system, still high FF of ~73.05% indicates that our thin Cu is enough to restrict loss of photogenerated carriers during collection and lateral transport steps. Thick Cu in the opaque PeSC serves as a rear reflector, increasing the
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Fig. 3. Photovoltaic parameters for semitransparent PeSCs with controlled interface; (a) Cu top electrode system. (b) Ag top electrode system.
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Fig. 4. (a) Representative J-V characteristic of the optimized opaque & semitransparent PeSCs. (b) Histograms of PCE for optimized semitransparent PeSCs.
Table 1 Photovoltaic characteristics of optimized opaque & semitransparent PeSCs.
Cu Ag Cu Ag
(40 nm) e Opaque PeSC (40 nm) e Opaque PeSC (8 nm) e Semi-transparent PeSC (8 nm) e Semi-transparent PeSC
Voc (V)
Jsc (mA/cm2)
Fill Factor
Efficiency
1.02 1.07 1.01 1.06
17.98 16.51 16.19 14.56
77.81 77.33 73.05 71.78
14.29 13.66 11.95 11.05
path length of incident light in the perovskite layer, and thus more harvesting of photons is possible. In contrast, the transmittance of 8 nm Cu leads to Jsc loss owing to nonaborbed photons passing through semitransparent Cu. Semitransparent PeSC with Ag also showed slightly lower performance comparing to opaque device due to same reasons with Cu based devices. As mentioned, Cu based devices showed slightly higher performance than Ag based system. Although we performed various characterization to verify the reason, we can not find any difference. But, as shown in the PCE statics (Fig. 4b) of devices fabricated in 4 different batches, Cu based semitransparent PeSCs showed higher efficiencies and narrower deviations than that of Ag based system. One of possible reason is better contact of Cu based device than that of Ag based system. Because Cu has a higher surface energy than Ag, interface between
PEIE modified PCBM and thin Cu is superior to that of Ag [46] Thus, better electron collection to Cu top electrode can be expected. Because reproducibility of device performance is critical issues in the commercialization stage to determine yield and overall production cost, minimizing the deviation in PCE and stability is important. Considering this point, Cu is better choice than Ag in thin metal based semitransparent PeSCs. As mentioned, the improvement of stability is equally important purpose to high efficiency in this study. Therefore, we monitored the change of PCE with exposure time in an ambient conditions (temperature of ~22 C and humidity of 40e50%) without any encapsulation and showed in Fig. 5a. Here, 3 devices per each configuration were tested in the same conditions. In case of Cu based system, PEDOT:PSS devices showed a gradual decline in PCE
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Fig. 5. (a) Change in PCEs during exposure to air without any encapsulation. (b) Cross sectional images of devices after stability test.
with time, while PCE of devices with NiOx was retained for 6 days without notable change. Replacement of hygroscopic and acidic PEDOT:PSS, which easily absorbs water from the environment and induces etch of the transparent electrode, with NiOX results in improved stability. Although very thin Cu of 8 nm was used as top electrode, this type of devices show excellent stability (80% PCE after 20 days). In contrast, some devices with Ag showed sharp decrease in PCE even though NiOx was used as HTL. After stability test, the cross-sectional SEM image of devices were characterized. Clear interface between each layer can be observed in the Cu based stable devices, while layered heterojunction of perovskite and PCBM was destructed in the case of Ag based device exhibiting poor stability.
4. Conclusion To demonstrate semitransparent PeSCs, opaque metal top electrode is replaced to semitransparent Ag & Cu electrode in
general ITO based device configurations. Although Cu has similar electrical conductivity with Ag, it is cheap and known to chemically stable even in direct contact with perovskite. Not only to achieve high efficiency but also to obtain excellent long-term device stability, Cu was carefully studied and compared with general Ag electrode. Further optimization of devices was systematically carried out by the control of interface of ITO/perovskite and PCBM/thin metal electrode, respectively. NiOx HTL based PeSCs showed higher efficiencies than conventional PEDOT:PSS based devices in both Ag and Cu systems due to more efficient hole collection resulted from well-matched energy level with VB of perovskite and effective blocking of electrons. When the PCBM was modified with PEIE, device performances were better than those without PEIE owing to improved contact barrier between the metal electrode and the PCBM. As a result, excellent Cu based semitransparent device with AVT of ~20%, achieving a Voc of 1.01 V, Jsc of 16.19 mA/cm 2, FF of 73.05%, and PCE of 11.95%, was successfully demonstrated. This type of device (using NiOx HTL instead of PEDOT:PSS and PCBM modified
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with PEIE) also showed excellent stability (80% PCE after 20 days). In contrast, some devices with Ag showed sharp decrease in PCE even though NiOx was used as HTL. Clear interface between each layer can be observed in the Cu based stable devices, while layered heterojunction of perovskite and PCBM was destructed in the case of Ag based device exhibiting poor stability. These results indicate that NiOx HTL and Cu top electrode is better choice to demonstrate air-stable as well as efficient semitransparent PeSCs. Acknowledgments This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (2018R1A2B6001206). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.jallcom.2019.05.051. References rez-Santiesteban, X. Mathew, [1] S.-H. Turren-Cruz, M. Saliba, M.T. Mayer, H. Jua L. Nienhaus, W. Tress, M.P. Erodici, M.-J. Sher, M.G. Bawendi, M. Gratzel, A. Abate, A. Hagfeldt, J.-P. Correa-Baena, Enhanced charge carrier mobility and lifetime suppress hysteresis and improve efficiency in planar perovskite solar cells, Energy Environ. Sci. 11 (2018) 78e86. [2] D.-Y. Son, S.-G. Kim, J.-Y. Seo, S.-H. Lee, H.J. Shin, D.H. Lee, N.-G. Park, Universal approach toward hysteresis-free perovskite solar cell via defect engineering, Journal of the American Chemical Society, J. Am. Chem. Soc. 140 (2018) 1358e1364. [3] W.S. Yang, B.W. Park, E.H. Jung, N.J. Jeon, W.C. Kim, D.U. Lee, S.S. Shin, J.W. Seo, E.K. Kim, J.H. Noh, S.I. Seok, Iodide management in formamidinium-leadhalideebased perovskite layers for efficient solar cells, Science 356 (2017) 1376e1379. [4] H.J. Snaith, Present status and future prospects of perovskite photovoltaics, Nat. Mater. 17 (2018) 372e376. [5] Best Research-Cell Efficiencies. https://www.nrel.gov/pv/assets/images/ efficiency-chart-20180716.jpg. [6] X. Zeng, T. Zhou, C. Leng, Z. Zang, M. Wang, W. Hu, X. Tang, S. Lu, L. Fang, M. Zhou, Performance improvement of perovskite solar cells by employing a CdSe quantum dot/PCBM composite as an electron transport layer, J. Mater. Chem. A. 5 (2017) 17499e17505. [7] M. Wang, Z. Zang, B. Yang, X. Hu, K. Sun, L. Sun, Performance improvement of perovskite solar cells through enhanced hole extraction: the role of iodide concentration gradient, Sol. Energy Mater. Sol. Cells 185 (2018) 117e123. [8] X. Zhao, S. Liu, H. Zhang, S.-Y. Chang, W. Huang, B. Zhu, Y. Shen, C. Chen, D. Wang, Y. Yang, M. Wang, 20% efficient perovskite solar cells with 2D electron transporting layer, Adv. Funct. Mater. 29 (2019) 1805168. n-Carmona, I. Zimmermann, E. Mosconi, X. Lee, [9] G. Grancini, C. Rolda D. Martineau, S. Narbey, F. Oswald, F. De Angelis, M. Graetze, Mohammad Khaja Nazeeruddin, One-Year stable perovskite solar cells by 2D/3D interface engineering, Nat. Commun. 8 (2017) 15684. [10] L. Meng, J. You, Y. Yang, Addressing the stability issue of perovskite solar cells for commercial applications, Nat. Commun. 9 (2018) 5265. [11] X. Qin, Z. Zhao, Y. Wang, J. Wu, Q. Jiang, J. You, Recent progress in stability of perovskite solar cells, J. Semicond. 38 (2017), 011002. [12] T. Zhou, M. Wang, Z. Zang, X. Tang, L. Fang, Two-dimensional lead-free hybrid halide perovskite using superatom anions with tunable electronic properties, Sol. Energy Mater. Sol. Cells 191 (2019) 33e38. [13] K.-T. Lee, L.J. Guo, H.J. Park, Neutral- and multi-colored semitransparent perovskite solar cells, Molecules 21 (2016) 475e495. [14] J. Sun, J.J. Jasieniak, Semi-transparent solar cells, J. Phys. D Appl. Phys. 50 (1e28) (2017), 093001. [15] Q. Xue, R. Xia, C.J. Brabec, H.-L. Yip, Recent advances in semi-transparent polymer and perovskite solar cells for power generating window applications, Energy Environ. Sci. 11 (2018) 1688e1709. [16] D.P. McMeekin, G. Sadoughi, W. Rehman, G.E. Eperon, M. Saliva, M.T. Horantner, A. Haghighirad, N. Sakai, L. Korte, B. Rech, M.B. Johnston, L.M. Herz, H.J. Snaith, A mixed-cation lead mixed-halide perovskite absorber for tandem solar cells, Science 351 (2016) 151e155. [17] B. Chen, Y. Bai, Z. Yu, T. Li, X. Zheng, Q. Dong, L. Shen, M. Boccard, A. Gruverman, Z. Holman, J. Huang, Efficient semitransparent perovskite solar cells for 23.0%-efficiency perovskite/silicon four-terminal tandem cells, Adv. Energy Mater. 6 (2016) 1601128 1e7. [18] C. Roldan-Carmona, O. Malinkiewicz, R. Betancur, G. Longo, C. Momblona, F. Jaramillo, L. Camacho, H.J. Bolink, High efficiency single-junction semitransparent perovskite solar cells, Energy Environ. Sci. 7 (2014) 2968e2973.
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