Organic‐inorganic halide perovskite materials have become a shining star in the photovoltaic field due to their unique properties, such as high absorption coefficient, optimal bandgap, and high defect tolerance, which also lead to the breathtaking increase in power conversion efficiency from 3.8% to over 22% in just seven years. Although the highest efficiency was obtained from the TiO2 mesoporous structure, there are increasing studies focusing on the planar structure device due to its processibility for large‐scale production. In particular, the planar p‐i‐n structure has attracted increasing attention on account of its tremendous advantages in, among other things, eliminating hysteresis alongside a competitive certified efficiency of over 20%. Crucial for the device performance enhancement has been the interface engineering for the past few years, especially for such planar p‐i‐n devices. The interface engineering aims to optimize device properties, such as charge transfer, defect passivation, band alignment, etc. Herein, recent progress on the interface engineering of planar p‐i‐n structure devices is reviewed. This review is mainly focused on the interface design between each layer in p‐i‐n structure devices, as well as grain boundaries, which are the interfaces between polycrystalline perovskite domains. Promising research directions are also suggested for further improvements.

Tin‐based perovskites with narrow bandgaps and high charge‐carrier mobilities are promising candidates for the preparation of efficient lead‐free perovskite solar cells (PSCs). However, the crystalline rate of tin‐based perovskites is much faster, leading to abundant trap states and much lower open‐circuit voltage (Voc). Here, hydrogen bonding is introduced to retard the crystalline rate of the FASnI3 perovskite. By adding poly(vinyl alcohol) (PVA), the OH…I− hydrogen bonding interactions between PVA and FASnI3 have the effects of introducing nucleation sites, slowing down the crystal growth, directing the crystal orientation, reducing the trap states, and suppressing the migration of the iodide ions. In the presence of the PVA additive, the FASnI3–PVA PSCs attain higher power conversion efficiency of 8.9% under a reverse scan with significantly improved Voc from 0.55 to 0.63 V, which is one of the highest Voc values for FASnI3‐based PSCs. More importantly, the FASnI3–PVA PSCs exhibit striking long‐term stability, with no decay in efficiency after 400 h of operation at the maximum power point. This approach, which makes use of the OH…I− hydrogen bonding interactions between PVA and FASnI3, is generally applicable for improving the efficiency and stability of the FASnI3‐based PSCs.

The poor crystalline quality of tin-based perovskite films with unfavorable trap states is the biggest challenge to achieve highly efficient tin-based perovskite solar cells. Here, we reveal the surface-controlled growth of FASnI 3 perovskites and further precisely control the crystallization process by reducing the surface energy of the solution-air surface with a tailor-made pentafluorophen-oxyethylammonium iodide (FOEI). A highly oriented and smooth FASnI 3 -FOEI perovskite film with longer carrier lifetime is achieved with a certificated efficiency of 10.16% from an accredited institute.

Lead-free tin halide perovskite solar cells (PSCs) have attracted great attention because of their low toxicity, ideal band gap, and high carrier mobilities. However, the efficiency and reproducibility of tin halide PSCs has been limited because of the facile oxidation of Sn 2+ to Sn 4+ . Herein, liquid formic acid (LFA) was introduced as a reducing solvent in the FASnI 3 (FA: formamidinium) perovskite precursor solution. Unlike solid reducing additives, the LFA solvent is volatile, so no residual LFA remained in the FASnI 3 perovskite film. Use of the LFA solvent resulted in production of the FASnI 3 perovskite film with high crystallinity, low Sn 4+ content, reduced background doping, and low electronic trap density. As a result, an efficiency of over 10% was obtained for lead-free tin halide PSCs with improved reproducibility.

Molecular design enabled reduction of interface trap density affords Molecular design enabled reduction of interface trap density affords highly efficient and stable perovskite solar cells with over 83% fill highly efficient and stable perovskite solar cells with over 83% fill factor factor

Tin halide perovskites are rising as promising materials for lead‐free perovskite solar cells (PSCs). However, the crystallization rate of tin halide perovskites is much faster than the lead‐based analogs, leading to more rampant trap states and lower efficiency. Here, we disclose a key finding to modulate the crystallization kinetics of FASnI3 through a non‐classical nucleation mechanism based on pre‐nucleation clusters (PNCs). By introducing piperazine dihydriodide to tune the colloidal chemistry of the FASnI3 perovskite precursor solution, stable clusters could be readily formed in the solution before nucleation. These pre‐nucleation clusters act as intermediate phase and thus can reduce the energy barrier for the perovskite nucleation, resulting in a high‐quality perovskite film with lower defect density. This PNCs‐based method has led to a conspicuous photovoltaic performance improvement for FASnI3‐based PSCs, delivering an impressive efficiency of 11.39 % plus improved stability.

Dihydronaphthyl-based [60]fullerene bisadduct derivative, NC(60)BA, was synthesized at mild temperature in high yield. NC(60)BA not only possesses a LUMO energy level 0.16 eV higher than PC(61)BM but also has amorphous nature that can overcome thermal-driven crystallization. The fabricated P3HT:NC(60)BA-based polymer solar cells exhibit superior photovoltaic performance and thermal stability compared to PC(61)BM-based devices under the same conditions.

Fullerene bisadducts have emerged as promising electron‐accepting materials because of their ability to increase the open‐circuit voltage (VOC) of polymer solar cells (PSCs) due to their relatively high lowest unoccupied molecular orbital (LUMO) energy levels. It should be noted that the as‐prepared fullerene bisadducts are in fact a mixture of isomers. Here, the effects of fullerene bisadduct regioisomers on photovoltaic performance are examined. The trans‐2, trans‐3, trans‐4, and e isomers of dihydronaphthyl‐based [60]fullerene bisadduct (NCBA) are isolated and used as acceptors for P3HT‐based PSCs. The four NCBA isomers exhibit different absorption spectra, electrochemical properties, and electron mobilities, leading to varying PCE values of 5.8, 6.3, 5.6, and 5.5%, respectively, which are higher than that based on an NCBA mixture (5.3%), suggesting the necessity to use the individual fullerene bisadduct isomer for high‐performance PSCs.