@article{b16dfb8a3ce741ea958b0572c381955b,
title = "Perovskite/perovskite planar tandem solar cells: A comprehensive guideline for reaching energy conversion efficiency beyond 30%",
abstract = "Perovskite/perovskite tandem solar cells (Pk/Pk TSCs) have a substantial potential to outperform the Shockley-Queisser limit of single-junction solar cells. However, optimum material bandgap selection and device processability impede the progress in acquiring efficient Pk/Pk TSCs. The choice of charge transport/contact materials additionally has a significant influence on the photovoltaic performance of Pk/Pk TSCs. Hence, the actual fabrication of a two-terminal Pk/Pk TSC becomes tricky, which requires a detailed understanding of the underlying optical and electrical properties of the device. In this study, a wide bandgap (~1.72 eV) lead iodine-bromide (Pb–I–Br) and a narrow bandgap (~1.16 eV) tin lead-iodide (Sn–Pb–I) perovskite absorbers are considered as potential sub-cells for realizing highly efficient planar Pk/Pk TSCs. Furthermore, energetically associated hole and electron selective contacts are prepared by atomic layer deposition (ALD) of metal oxides. The optics of solar cells is investigated by three-dimensional finite-difference time-domain (FDTD) optical simulations, and finite element method (FEM) electrical simulations are exploited to determine realistic photovoltaic performance parameters. A comprehensive study is carried out to provide a complete guideline for the realization of energy conversion efficiency exceeding 30% in Pk/Pk TSCs.",
keywords = "ALD, Current matching, Metal oxides, Optics, Perovskite, Tandem solar cell",
author = "Hossain, {Mohammad Ismail} and Saleque, {Ahmed M.} and Safayet Ahmed and Ilhom Saidjafarzoda and Md Shahiduzzaman and Wayesh Qarony and Dietmar Knipp and Necmi Biyikli and Tsang, {Yuen Hong}",
note = "Funding Information: This work is financially supported by the Research Grants Council of Hong Kong, China (Project number: 152093/18E), the Hong Kong Polytechnic University (Project number: G-YBVG), the Hong Kong Polytechnic University Shenzhen Research Institute , Shenzhen, China (Grant Code: the science and technology innovation commission of Shenzhen ( JCY20180306173805740 )). N.B. acknowledges the financial support by the UCONN's Office of the Vice President for Research (OVPR) through the Research Excellence Program (REP). Funding Information: In this section, the narrow bandgap or the bottom single-junction PSC is optimized. The bottom PSC has a substantial influence on the JSC and the ECE of TSCs. An upper limit of the JSC in the Pk/Pk TSC can be predicted from the bottom cell JSC, which is 50% of the bottom cell JSC. It is assumed that the lower bandgap perovskite can exhibit high JSC. Several studies reported remarkable ways to lower the bandgap by using the Pb/Sn binary perovskite (MAPb1-xSnxI3) alloys [ 52?54]. In this study, we propose the use of MASnPbl perovskite (Eg~1.16 eV) absorber, which is sandwiched between AZO and NiO/ZnO charge transporting layers. A schematic cross-section of the investigated bottom PSCs is visualized in Fig. 4(a). The absorber thickness plays an essential role in maximizing QE and JSC, since photon absorptions only in the absorber layer contribute to the JSC of the solar cell. Thus, the bottom cell is optimized by varying the thickness of the perovskite absorber ranging from 200 nm to 2000 nm with a step size of 200 nm, while maintaining the realistic diffusion length of the material system. Solar cell with such a low bandgap perovskite allows attaining broadband photon absorptions ranging from 300 nm (UV region) to 1100 nm (Infrared region). The influence of absorber layer thickness is investigated on absorption and QE, as shown in Fig. 4(b). Furthermore, the absorptions and QEs spectra for 200, 600, 1000, 1400, and 1800 nm absorber thicknesses are also provided in the Supporting Information (Fig. S5). A maximum QE/absorption approaching unity is found to be exhibited around 550 nm, while the thickness has almost to influence on QE below 650 nm wavelength. Such a high absorption is attained due to the device design with efficient metal oxide front contact with high optoelectronic properties, allowing to obtain an effective light coupling in the solar cell. Then the absorption and QE in the longer wavelengths (>700 nm) almost linearly increase with the increase of the thickness. The QEs also exhibit zero absorption just after 1100 nm wavelength. Overall, the solar cell pronounces very low reflection and parasitic losses, which improves the JSCs. This is also confirmed by the investigated power density profiles and field distributions as provided in the Supporting Information (Fig. S6 and Fig. S7). Fig. 4(d) presents the corresponding JSCs of the bottom PSCs. The JSC is distinctly increased from 30.5 mA/cm2 to 37.5 mA/cm2, resulting in almost 19% enhancement by varying the thickness from 200 nm to 2000 nm.In the optimized TSC, only ~4.5% of total absorption is accounted into parasitic losses for the ALD-grown AZO layer used in the tandem solar cell, which is very low. However, reflection losses are considerably yet high (~12%), which can be mitigated further by applying surface and interface engineering. Nevertheless, this is not the scope of the current study. The optics of the Pk/Pk TSC under matching condition is also supported by the power density and electric field distribution plots. Power densities of the Pk/Pk TSC for a monochromatic wavelength of 400 nm, 500 nm, 700 nm, 800 nm, 900 nm, and 1100 nm are illustrated in Fig. 8(b?g) along with the schematic cross-section in Fig. 8(a). Corresponding electric field distribution plots are provided in the Supporting Information (Fig. S12). For short wavelengths (<400 nm), most photons are absorbed by the vicinity of front contact and top perovskite absorber, while a fraction of the light propagates to the bulk of the absorber.Detailed investigations on the electronic properties of the Pk/Pk TSC are provided in the Supporting Information (Figs. S13?S17), where the influence of band structures, doping densities, electrostatic potential, electron current density, and hole current density are broadly explained. The calculated JSC of the Pk/Pk TSC through electrical simulations is slightly lower than the JSC from optical simulations due to the electrical effects. However, findings in both optical and electrical simulations are very much comparable, which confirms the accuracy of our provided investigations. Furthermore, investigated photovoltaic performance parameters are compared with the theoretical upper limit (SQ limit) of solar cells, which are demonstrated in Table 2. Table 2 represents a ratio between photovoltaic performance as tabulated in Table 1 and the Shockley-Queisser (SQ) limits, which indicates the proximity of the investigated solar cells with respect to the SQ limit. The calculated SQ limit for the single-junction PSC is provided in the Supporting Information (Fig. S18) [27]. The upper limit of the 2T Pk/Pk TSC is realized from the detailed balance theory (an extended SQ limit for the double-junction solar cell), as shown in Fig. 1(c). More details on how to calculate the upper limit for both single-junction and double-junction (tandem) solar cells are provided in our previously published works [28].Furthermore, the SQ limit of JSC, VOC, FF, and ECE of the single-junction solar cells are provided in the Supporting Information (Fig. S18). It is observed that the investigated electrical parameters (VOC and FF) are approaching (?90%) their theoretical limit, where JSCs ranging from 80 to 90% of its maximum value. Nevertheless, it is assumed that the utilization of the potential surface and interface engineering can further amplify JSCs. The ECE of investigated solar cells is continuing between 70 and 80% of its theoretical value. Hence, it is believed that the provided approach has a great potential to cross the SQ limit of the single-junction solar cells and will give a pathway to realize next-generation high-efficiency solar cells.This work is financially supported by the Research Grants Council of Hong Kong, China (Project number: 152093/18E), the Hong Kong Polytechnic University (Project number: G-YBVG), the Hong Kong Polytechnic University Shenzhen Research Institute, Shenzhen, China (Grant Code: the science and technology innovation commission of Shenzhen (JCY20180306173805740)). N.B. acknowledges the financial support by the UCONN's Office of the Vice President for Research (OVPR) through the Research Excellence Program (REP). Publisher Copyright: {\textcopyright} 2020 Elsevier Ltd",
year = "2021",
month = jan,
doi = "10.1016/j.nanoen.2020.105400",
language = "English",
volume = "79",
journal = "Nano Energy",
issn = "2211-2855",
publisher = "Elsevier B.V.",
}
