###### Abstract  
In the competition of solar cell efficiency, besides top-performance multijunction cells, tandem cells based on perovskites are also breaking efficiency records to enter into the 30% range. Their design takes advantage of the rapid development of perovskite cells, and the good sharing of the available spectrum between the perovskite, absorbing at short wavelengths, and the c-Si or similar lower band gap material, working at longer wavelengths. In this paper, we present a novel tandem solar cell that combines crystalline silicon (c-Si) and perovskites cells. We analyzed the device with computational electromagnetism based on the finite element method. Our design arranges the perovskite solar cell as a multilayer 1D grating, which is terminated with a gold thin film (top metallic contact). This multilayer nanostructure is placed on top of the c-Si cell and a thin protective dielectric layer of aluminum nitride covers the whole device. The short-circuit current of the perovskite cell is maximized by maintaining the current-matching conditions with the output from the c-Si cell. This optimization considers the geometrical parameters of the grating: period and thickness of the active layer of the perovskite cell. We compared the simulated short-circuit current of this device to the planar tandem solar cell with indium in oxide (top contact). The comparison shows a slight circumvent, around 3%, of our device's performance. Moreover, it has the potential capability to circumvent postprocessing procedures used with transparent contact oxides, which can reduce the device's final efficiency. Furthermore, our proposed design can take advantage of photolithographic and nanoimprint techniques, enabling large-scale production at a relatively low cost. tandem perovskite/c-Si solar cell; nanostructured metallic top contact; optical modeling; computational electromagnetism +
Footnote †: journal: Applied Optics Complutense Group, Faculty of Optics and Optometry, University Complutense of Madrid, 28037 Madrid, Spain; [EMAIL_ADDRESS]  
Footnote †: journal: Applied Optics Complutense Group, Faculty of Optics and Optometry, University Complutense of Madrid, 28037 Madrid, Spain; javier. the mechanical stack of the two junctions and require a lower number of contacts [7]. Here, we selected the two-terminal configuration because it simplifies simulations with computational electromagnetism tools. Nevertheless, the analysis proposed here can be implemented for other configurations. Perovskite/crystalline silicon (c-Si) is one of the most promising combinations for this kind of device with a potential efficiency \(>\)30% [8]. In fact, perovskite is an appropriate choice for the upper cell. It presents a large absorption at shorter wavelengths and its photovoltaic conversion efficiency has rapidly increased in the last few years [9]. Most devices include an indium tin oxide (ITO) top contact: a transparent conductive oxide (TCO) [10]. This material is widely used because it has a high average transmission at the visible region, T \(\approx\) 85% [11], and low sheet resistance, \(\sim\)8-12 \(\Omega\)/\(\square\)[12]. This is why it is also widely used as a top contact in other optoelectronic devices, such as light-emitting diodes [13] and touch displays [14]. However, ITO has some drawbacks: there is a limited supply of indium in the Earth's crust, resulting in a high-cost production for large-scale applications [15]. ITO is also a hard and rigid material that cracks when bent. This degrades its performance in flexible devices. Moreover, high efficiency ITO devices require postprocessing steps, including annealing at high temperatures which affects the perovskite solar cell layers [16, 17]. To replace ITO, there are many structures being investigated: nanowires [18], carbon nanotubes [19], thin metal films [20], metal grids prepared by lithography [21], or printing [22], and metal nanohole arrays [23].