Fig. 1.  (a) Schematic diagram of the monolithic perovskite/silicon tandem solar cell based on a double-side textured flexible silicon heterojunction solar cell, scanning electron microscopy (SEM) cross-sectional image on the right shows the perovskite top-cell layers and ~60 μm thin silicon bottom cell. (b) Schematic diagram of conventional atomic layer deposition (ALD) process for compact SnO_x layer (left, purging time of 10 s), and loose SnO_x layer (right, with significantly reduced purging time) under a chemical vapor deposition (CVD)-like growth mode. (c) Geometric phase analysis (GPA) of high-resolution transmission electron microscope (TEM) images of compact and loose SnO_x layer. (d) Steady photoluminescence (PL) spectra of the perovskite (PVSK)/C₆₀/SnO_x samples and the PVSK/C₆₀/SnO_x/transparent conductive oxide (TCO) samples, where SnO_x layers are deposited under different purging times. (e) Current density−voltage (J−V) curve and the maximum power output point of one 60-μm thick tandem device, measured by National Renewable Energy Laboratory (NREL) using the asymptotic maximum power scan method. (f) Current−voltage (I−V) curves of one M6 wafer-sized (~260 cm²) lightweight tandem device, certified by Fraunhofer ISE CalLab, the inset shows the weight of a complete M6-sized tandem solar cell with 4.386 g and derived a 1.77 W/g power-to-weight ratio. (g) Photograph of flexible perovskite/silicon tandem devices, which can be folded with a bending radius of 15 mm (25 mm). (h) Thermal cycling tests (−40 to 85 °C) for the encapsulated tandem solar cells. (i) Power conversion efficiency (PCE) evolution for wafer-sized lightweight unencapsulated tandem devices as a function of bending cycles, the inset shows PL mapping images of the tandem devices before and after bending test^[22].

Fig. 2.  (a) Sketch of the flexible perovskite/silicon tandem with cerium and hydrogen co-doped indium oxide (ICO:H) recombination layer (RL) and in-situ annealed zinc-doped indium oxide (IZO) front electrode. (b) Cross-section SEM images of the flexible tandem; high-angle annular dark-field scanning transmission electron microscopy STEM (HAADF) images with energy-dispersive X-ray spectroscopy (EDX) mapping of wide bandgap perovskite top cell and ICO:H RL on submicron-textured SHJ bottom cell. (c) Density-functional theory (DFT) calculation of absorption energy and charge transfer characteristics at the interface between 4-(3,6-Dimethyl-9H-carbazol-9-yl)butyl phosphonic acid (Me-4PACz) and ICO:H. (d) Kelvin-probe force microscopy (KPFM) images of Me-4PACz anchored ICO:H or ITO. (e) The ultraviolet−visible−near infrared (UV−vis−NIR) spectroscopy of IZO with and without in-situ annealing. (f) The proportion In−O, C−O and −OH species extracted from X-ray photoelectron spectroscopy (XPS) analysis. (g) J−V curves of the control (ITO RL (20 nm), non-annealed IZO) and target (ICO:H (10 nm), annealed IZO) flexible perovskite/silicon tandem with an aperture area of 1.0 cm², calibrated at China Photovoltaic Testing Center (NPVT). (h) Maximum power point (MPP) tracking of the encapsulated flexible tandems under continuous 1-sun illumination in N₂ atmosphere. (i) Bending tests of flexible tandems under compressive stress with a bending radius (R_b) of 17.6 mm (the insert)^[23].