In recent years, the power conversion efficiency (PCE) for single-junction perovskite solar cells (PSCs)^{[1, 2]} has reached 25.7%, approaching the Shockley-Queisser limit (S-Q limit). Further enhancing efficiency is challenging. Tandem solar cells offer an effective way to further increase the efficiency beyond S-Q limit. Currently, perovskite/silicon tandem solar cells (TSCs)^[3-5] have achieved a PCE of 31.3%^[6]. However, the complicated preparation processes and high cost hinder their commercialization application. In contrast, thin-film perovskite/organic TSCs have the advantages of solution processability, low cost, and flexibility, making them to be promising candidates for the next-generation photovoltaic technology.

The idea of combining perovskite with photoactive organics was first proposed by Ding et al.^[7] in 2014, and they designed the first integrated solar cell structured with ITO/PEDOT:PSS/CH₃NH₃PbI₃/(PDPP3T:PC₆₁BM)/Ca/Al. A bulk heterojunction layer composed of PDPP3T:PC₆₁BM blend was inserted into a PSC structure, which broadened the spectral response to 970 nm. Though this structure is not a tandem structure, as there is no interconnecting layer (ICL), it still inspired the attempts to combine PSCs with organic materials to broaden the photoresponse. After that, Jen et al.^[8] fabricated the first perovskite/organic TSCs with PBSeDTEG8 as the active layer of front subcell and CH₃NH₃PbI₃ as the active layer of rear subcell. The tandem cells gave a PCE of 10.23%. However, the structure with organic solar cell (OSC) as the front cell would cause serious deposition problem, as the high temperature and solvent treatment during the processing of perovskite layer could damage the already deposited OSC. Russell et al.^[9] designed a ~90 nm thick perovskite-based front subcell and a ~100 nm thick polymer-based rear subcell, yielding a PCE of 15.9%. This structure allows long wavelength light to pass through PSC and to be utilized by the OSC (Fig. 1). Also, it avoids the damage to organic layer during processing.

After years of exploration, many effective strategies have been proposed to improve the photovoltaic performance of perovskite/organic TSCs, including broadening the light absorption, optimizing the ICL, and passivating the bulk and interfacial defects. The PCE for perovskite/organic TSCs has reached 23.1%^[10]. The PCE progress for perovskite/organic TSCs is illustrated in Fig. 2, and the device details can be found in Table 1.

Broadening the absorption spectra. Perovskite and organic materials absorb photons of short and long wavelength, respectively. The absorption spectra for both perovskite and organic materials in early TSCs are overlapping, leading to insufficient sunlight utilization. Therefore, designing wider-bandgap perovskite and narrower-bandgap organic materials to enhance sunlight absorption is an effective approach to improve the performance of TSCs. In 2019, Ding et al.^[11] developed a 2T tandem solar cell with CsPbI₂Br cell as the front cell with high response to light before 650 nm and PTB7-Th:CO_i8DFIC:PC₇₁BM ternary cell as the rear cell absorbing light at 650–1050 nm. The TSC offered a 15.04% PCE. In 2020, Cao et al.^[12] used a wide-bandgap (WBG) CsPbI₂Br front cell in series with a narrow-bandgap (NBG) PM6:Y6 rear cell and obtained a TSC with a 18.4% PCE. The thickness control of perovskite layer is vital for the optimal utilization of the solar spectrum. If it is too thin, the short-wavelength photons cannot be absorbed completely; when it is too thick, the transmittance would be greatly reduced. In 2021, Ding et al.^[13] further enhanced the efficiency of inorganic perovskite/organic TSCs to 20.18% by integrating a WBG CsPbI₂Br front cell and a D18:Y6 organic rear cell. Since then, scientists have focused on varying the ratio of I and Br in CsPbI₂Br to tune the bandgap, pushing the PCE to more than 21%^{[14, 15]}. Organic–inorganic hybrid perovskite was also used in perovskite/organic TSCs in recent years. In 2020, Yang et al.^[16] proposed a semi-empirical model to select the optimum combination of materials for perovskite/organic TSCs. The optimal combination was FA_0.8MA_0.02Cs_0.18PbI_1.8Br_1.2 (1.77 eV) and PBDBT-2F:Y6:PC₇₁BM (1.41 eV), and the TSCs gave high PCE (certified PCE of 19.54%) and high reproducibility. In 2022, Cao et al.^[17] used MAPbI₂Br doped with Pb(SCN)₂ front cell and an organic rear cell to make a TSC with over 20.03% PCE. In 2022, Li et al.^[18] synthesized a small molecule acceptor BTPV-4Cl-eC9 to expand the photoresponse to infrared region, and they combined PTB7-Th:BTPV-4Cl-eC9 rear cell with FA_0.6MA_0.4Pb(I_0.6Br_0.4)₃ front cell to make high-performance TSCs with a PCE of 22%.

Optimizing the ICL. The ICL connects two subcells to achieve electron and hole recombination and protects the front cell from being destroyed during the deposition process of the rear cell. A good ICL possesses good transmittance, chemical stability, and low resistive losses. In 2016, C₆₀-SB/Ag/MoO₃ was used as the ICL^[9], which alleviated the damage of thermal annealing and chemical treatment to polymer front cells during the process of perovskite layer. In 2020, Yang et al.^[16] demonstrated a simplified ICL structure by using a thin MoO_x/Ag/BCP layer, yielding low optical loss and high reproducibility. In 2021, Ding et al.^[13] used PTAA/MoO₃/Au/ZnO as the ICL. The thin Au layer (1 nm) reduces the charge-transport barrier and contributes to the high PCE of 20.18%. The above ICLs adopted a thin layer of metal inserted between two charge-transport layers. However, despite the excellent conductivity of metal, it is not transparent, causing optical losses. Very recently, Riedl et al.^[10] inserted an ultrathin InO_x layer (1.5 nm) made by atomic layer deposition (ALD) to eliminate the Schottky barrier between the low-workfunction SnO_x and the high-workfunction MoO_x. InO_x avoided the use of metal layer and the optical loss is negligible, greatly improving the photovoltaic performance of TSCs. Hou et al.^[19] developed C₆₀/BCP/IZO/MoO_x ICL. IZO can improve carrier recombination and minimize the optical losses at the same time, yielding a certified PCE of 22.94%. Cao et al.^[17] investigated the influence of ICL thickness on the performance of TSCs. Thin ICL may lead to poor device performance due to inefficient recombination, while thick ICL can lead to inefficient electron extraction, lower subcell EQE and device efficiency, so the ICL thickness needs to be optimized. Overall, an excellent ICL layer should be chemically inert, conductive, and transparent, providing sufficient recombination sites between the front and rear subcells.

Passivating bulk and interfacial defects. A large number of defects exist in polycrystalline perovskite films, especially at the interface between ICL and perovskite layer, which will cause severe nonradiative recombination. It leads to large V_oc loss and undermines the long-term stability of perovskite front cell^[13]. So, interfacial passivation is essential for improving the performance of TSCs. In 2020, Jen et al.^[20] used phenmethylammonium bromide to passivate the WBG perovskite layer Cs_0.1(FA_0.6MA_0.4)_0.9Pb(I_0.6Br_0.4)₃ and the TSCs gave a PCE of 15.13%. In 2021, Cao et al.^[21] reduced defects at grain boundaries and improved phase stability through compositional engineering. FA⁺ was incorporated, and MA_0.9FA_0.1PbI₂Br doped with Pb(SCN)₂ presented homogeneous crystallization and reduced SCN^–-induced PbI₂ excessive aggregation. The TSCs with PM6:CH1007 subcell gave a PCE of 21%. In 2022, Li et al.^[18] introduced organic cation ClFA⁺ to FA_0.6MA_0.4Pb(I_0.6Br_0.4)₃ film to passivate bulk defects in perovskite films. A high V_oc of 1.25 V and a high FF of 83.0% for the perovskite front cell were achieved. In 2022, Riedl et al.^[10] used large organic cation-based halides to passivate perovskite surface by forming a two-dimensional (2D) layer. The valence band was lowered to prevent holes from reaching the electron extraction layer, and an efficiency of 23.1% was achieved. In 2022, Hou et al.^[19] used nickel oxide hole-transport layer passivated by phenylphosphonic acid, and obtained a certified efficiency of 22.94%.

In short, perovskite-organic TSCs already demonstrate a record PCE of 23.1%. Further work should focus on long-term stability, reproducible and reliable fabrication, and low cost, which are very important for future commercialization.

Acknowledgements

L. Ding thanks the National Key Research and Development Program of China (2022YFB3803300), the open research fund of Songshan Lake Materials Laboratory (2021SLABFK02), and the National Natural Science Foundation of China (21961160720) for financial support.