Progress toward perovskite solar cells (PSCs) with higher efficiency and stability requires further enhancing the light absorption, reducing the carrier recombination at defects and interfaces, improving the charge carrier separation, and minimizing the energy loss crossing the interfaces. Interface passivation and device structure optimization are the main strategies to improve efficiency and device stability. In this work, we report a simple method to combine the two strategies, i.e., realize the pervoskite heterostructure and the passivation layer simultaneously in a single two-dimensional (2D) PEA2PbI4–three-dimensional (3D) MAPbI3 composite perovskite interfacial layer. We use a mixed solution of methylammonium iodide (MAI) and phenethylammonium iodide (PEAI) to react with the intentionally introduced excess PbI2 in the Cs0.05(MA0.15FA0.85)0.95Pb­(I0.85Br0.15)3 (CsMAFA) perovskite layer and form the MAPbI3/CsMAFA heterostructure and the 2D PEA2PbI4 perovskite passivation layer. Systematic investigations show that the MAPbI3/CsMAFA heterostructure can enhance the light absorption and the charge carrier separation at the interfaces, and thus improve the short-circuit current (J sc) of solar cells. The 2D PEA2PbI4 perovskite layer can effectively passivate the interfacial defects and improve the fill factor (FF) and open-circuit voltage (V oc). The working mechanisms of MAI and PEAI treatment are also discussed. This work offers a promising path for the fabrication of highly efficient and stable PSCs.

Modification of the electron transport layers is extremely important for improving the efficiency and stability of perovskite solar cells (PSCs) due to its critical role in the electron extraction and crystallization of perovskite layers. Na2S has been reported as an effective dopant and a surface-modification layer as well for a compact TiO2 layer. This paper further investigated the distribution and the working mechanism of sulfur ions via comparing the properties of Na2S-doped and Na2S-coated TiO2 layers and the corresponding devices. A simple deionized-water rinsing process was introduced for its removal capability of Na and S elements in compacted TiO2 layers. From the characterization results of TiO2 layers after rinsing and corresponding PSCs, we can draw the conclusion that Na+ and S2– are mainly located on the TiO2 surface and grain boundaries in the TiO2 layers, and only a very small amount of S elements participates in the bonding with the TiO2 crystal lattice of the Na2S-doped TiO2 layers. This work provides some insights on the understanding of the mechanism of S2– passivation in PSCs and may stimulate further investigation on defect passivation in the perovskite research community.

The low electrical conductivity and the high surface defect density of the TiO2 electron transport layer (ETL) limit the quality of the following perovskite (PVK) layers and the power conversion efficiency (PCE) of corresponding perovskite solar cells (PSCs). Sulfur was reported as an effective element to passivate the TiO2 layer and improve the PCE of PSCs. In this work, we further investigate the effect of chemical valences of sulfur on the performance of TiO2/PVK interfaces, CsFAMA PVK layers, and solar cells using TiO2 ETL layers treated with Na2S, Na2S2O3, and Na2SO4, respectively. Experimental results show that the Na2S and Na2S2O3 interfacial layers can enlarge the grain size of PVK layers, reduce the defect density at the TiO2/PVK interface, and improve the device efficiency and stability. Meanwhile, the Na2SO4 interfacial layer leads to a smaller perovskite grain size and a slightly degraded TiO2/PVK interface and device performance. These results indicate that S2– can obviously improve the quality of TiO2 and PVK layers and TiO2/PVK interfaces, while SO4 2– has little effects, even negative effects, on PSCs. This work can deepen the understanding of the interaction between sulfur and the PVK layer and may inspire further progress in the surface passivation field.