Fig. 1 Device structures of 2T tandem solar cells based on p-i-n structured perovskite subcells

Fig. 2 Structures and device performance of 2T tandem solar cells based on thin metal RLs (a) Device structure and cross-section SEM image of perovskite-organic 2T tandem solar cell with Ag as the RL[12]; (b) Device structure of perovskite-organic 2T tandem solar cell with Ag nanoparticles as RL and the corresponding simulation result of the light field distribution[30]; (c) Device structure of perovskite-perovskite 2T tandem solar cell with Au as RL(left), J-V curves of devices without and with ultrathin Au layer as RL(middle), and J-V curves of a large-area device (1.05 cm2) with inset showing the digital photo of the large-area device (right)[32] BCP: Bathocuproine

Fig. 3 Photoelectric properties of ITO nanocrystals and schematic diagram of preparation process of corresponding 2T tandem solar cell (a) X-ray diffraction pattern of ITO nanocrystals[37]; (b) Dark J-V curves of devices based on ICLs without RL, with Au as RL, and with ITO as RL[37]; (c) Optical absorptance curves of ICLs without RL, with Au and ITO as RL[37]; (d) Schematic diagram of preparation process of the perovskite-silicon tandem solar cell[39]. E-NiOx: NiOx nanocrystals which are dispersible in ethanol; AR: Anti-reflection film; SHJ: Silicon heterojunction. Colorful figures are available on website

Fig. 4 IZO as RL in 2T tandem solar cells and the corresponding characterization[40] (a) Schematic diagram showing the p-i-n structured perovskite-organic 2T tandem solar cells (The dashed grey frame indicates the ICL region and the design of ICLs with four types of RL are depicted in the frame); (b) J-V curves (reverse scan) of devices using IZO-based ICLs with different thicknesses; Transmission electron microscopy image of the (c) 4 nm-thickness IZO and (d) 1 nm-thickness Ag on BCP. CRL: Carrier recombination layer; OPV: Organic photovoltaic. Colorful figures are available on website

Fig. 5 Perovskite-perovskite 2T tandem solar cell and corresponding device performance before and after the simplification of ICL[44] (a) Schematic diagrams of 2T tandem solar cells based on typical structured ICLs of C60/SnO2−x/ITO/PEDOT:PSS (left) and the simplified ICLs of C60/SnO2−x (right); (b) J-V curves of devices based on various ICLs. NBG: Narrow bandgap; WBG: Wide bandgap. Colorful figures are available on website

Fig. 6 InOx RL-based 2T tandem solar cells and their photoelectric properties (a) Schematic of perovskite-organic 2T tandem solar cell with InOx or Ag as RL[45]; (b) J-V characteristics of tandem solar cells with varied thickness (number of ALD cycles) of InOx layers[45]; (c) Optical transmittance of InOx, SnOx/InOx/MoOx, and SnOx/1 nm Ag/MoOx[45]; (d) Resulting External Quantum Efficiency (EQE) spectra of the organic rear subcell of tandem solar cells with InOx or Ag as RL[45]. Colorful figures are available on website

Fig. 7 2T tandem solar cells with PEDOT:PSS or GO as RL (a) Schematic of the as-prepared 2T bottom-up perovskite-perovskite tandem solar cell and chemical structures of spiro-OMeTAD, PEDOT:PSS, PEI and PCBM[47]; (b) Schematic diagram and long-term photo-stability for the perovskite-perovskite tandem solar cell[24]