The development of solar cells has made a great progress with new materials such as perovskites, copper indium gallium selenide (CIGS), organic/polymers in recent decades [1], [2], [3], [4], [5], [6], [7], [8], [9]. Especially, for perovskite, the recorded power conversion efficiency (PCE) increases rapidly, exhibiting the latest certified PCE of 25.7% for single junction solar cell [10]. When the record efficiency of single junction solar cells approaches the Shockley–Queisser limit, multi-junction tandem architecture is considered as an effective way to break the limit [11]. A record efficiency of 29.8% for perovskite/silicon tandem solar cell has also been achieved recently [10]. Meanwhile, black phosphorus (BP), the emerging active material, has been proved to be a promising active layer used in solar cells [12], [13], [14], [15]. The feasibility of BP as active layers in single junction thin-film solar cells has been studied extensively due to its high optical absorption coefficient ranging from 0.3μm to 1.2μm. Simultaneously, excellent absorption coefficient of BP in short wavelength range from 0.3μm to 0.5μm makes it an attractive candidate as active layer of the top sub-cell in the tandem solar cells together with Si as bottom sub-cell [16], [17], [18], [19]. However, to the best of our knowledge, BP has not been fully studied as top sub-cell in tandem solar cells. It is highly desired to explore the application of BP in Si-based tandem solar cells. On the other hand, optical designs are often employed to increase the absorption of tandem solar cell with light trapping effects [20]. Top surface texturing with nanostructures, so called antireflective layer, could help to reduce the surface reflection loss [21]; Activelayer texturing could help to enhance light absorption in active layer directly with excited optical modes [22]; Bottom layer texturing could help to absorb the light more efficiently by reflecting light back to active layer [23]. In our previous publications, we also shown that structured configurations make significantly enhancement to solar cell performance in our previous work [24], [25], [26]. However, the light management strategy is quite different for tandem solar cells. Solar energy of short wavelengths is designed to absorbed by top sub cell while solar energy of long wavelengths is preferred to be absorbed in bottom sub cell after passing through the top sub cell. It is worth to note that for tandem solar cells, interlayers inserted between top and bottom sub cells is also considered as an efficient way to enhance light absorption by reflecting light of short wavelengths back to top sub cell and letting light of long wavelengths transmit to bottom sub cell [27], [28], [29], [30], [31].

In this paper, we design a tandem solar cell with BP and c-Si as active layers of top and bottom sub-cells respectively and exam the optical designs such as one dimensional photonic crystal interlayers (ILs) and nanopyramid antireflective layer in the proposed BP/C-Si tandem cell. A short-circuit current (Jsc) of 15.52 mA/cm2 is achieved with the proposed tandem solar cell. Tandem solar cell with optimized one dimensional photonic crystal ILs show significant enhancement of light absorption owing to the reduced reflection loss. By inserting ILs between top and bottom sub-cells, we achieve a short-circuit current (Jsc) of 18.50 mA/cm2 which is increased by 2.98 mA/cm2 (19.2%) compared to that of the tandem cell without ILs.