In about 11 years, the photoelectric conversion efficiency (PCE) of perovskite solar cells (PSCs) has increased from 3.8% in 2009 [7] to 25.7% in 2022. [8] The electron transport layer (ETL) is an important component of PSCs and plays an important role in transporting electrons and blocking holes, as well as eliminating electrical shunting between the transparent electrode/perovskite interface. [9,10] The charge extraction ability and interfacial band alignment of ETL have important effects on the device performance of PSCs. [11][12][13] Insufficient charge extraction between the perovskite layer and the ETL can lead to interfacial charge accumulation, which negatively affects device performance and stability. [14] However, due to some inherent limitations of material properties, single-layer ETL is difficult to satisfy the requirements for the rapid development of PSCs. For example, Titanium oxide (TiO 2 ) is one of the most widely used ETLs in PSCs due to its good optical transparency and chemical stability. However, its electron mobility and extraction capability are low leading to charge accumulation at the ETL/perovskite interface and then resulting in reduced efficiency and stability. [15][16][17] Fang et al. [18] first used SnO 2 as an ETL application in PSCs in 2015. You et al. [4] reported high-efficiency PSCs using tin oxide (SnO 2 ) as ETL with a lower energy level of the bottom of conduction band (E CBM ) which can facilitate electron transport and reduce the charge accumulation at the interface. Thus, SnO 2 has become an extremely popular ETL due to its wide bandgap, high transmittance, high carrier mobility, and good chemical stability. [19,20] However, SnO 2 based PSCs exhibit relatively low electroluminescence external quantum efficiency (EQE) values, and hence higher open-circuit voltage (V OC ) loss due to the lower E CBM . [21] It can be seen that there is a dilemma in E CBM for single-layer ETL which goes ill with V OC when is beneficial to electron transportation and vice versa. Moreover, other dilemmas are often encountered when the potentials in ETL are extremely dug to further improve the performance of PSCs. For instance, highly rough superstrate is applied to improve J SC of PSCs by enhancing light scattering, and then the ETL grown on this kind of superstrate need to be thick enough to get a good Combining two kinds of electron transport layer (ETL) which have complementary advantages into a bilayer structure to form a bilayer ETL is an effective way to transcend inherent limitations of single-layer ETL, which is very helpful in the development of perovskite solar cells (PSCs). In this work, a strategy is proposed to break constraints on the application of the staggered bilayer ETL in high-efficiency PSC, namely utilizing a built-in field to overcome the dilemma in E CBM making it possible to improve V OC and FF simultaneously by tuning the Fermi level of ETLs properly. According to the strategy, a bilayer ETL structure comprised of C-TiO 2 and SnO 2 layer and corresponding Li-doping ...

Although the hot-casting (HC) method can obtain efficient quasi-2D perovskite solar cells, this method cannot effectively control the uniformity of the thin film, and the high preheating substrate temperature will...