Multi-Functional Regulation on Buried Interface for Achieving Efficient Triple-Cation Perovskite Solar Cells.

Mixed-cation perovskite solar cells (PSCs) have attracted much attention because of the advantages of suitable bandgap and stability. It is still a challenge to rationally design and modify the perovskite/tin oxide (SnO2) heterogeneous interface for achieving highly efficient and stable PSCs. Herein, a strategy of one-stone-for-three-birds is proposed to achieve multi-functional interface regulation via introducing N-Chlorosuccinimide (NCS) into the solution of SnO2: i) C═O functional group in NCS can induces strong binding affinity to uncoordinated defects (oxygen vacancies, free lead ions, etc) at the buried interface and passivate them; ii) incomplete in situ hydrolysis reactions can occur spontaneously and adjust the pH value of the SnO2 solution to achieve a more matchable energy level; iii) effectively releasing the residual stress of the underlying perovskite. As a result, a champion power conversion efficiency (PCE) of 24.74% is achieved with a device structure of ITO/SnO2/Perovskite/Spiro-OMeTAD/Ag, which is one of the highest values for cesium-formamidinium-methylammonium (CsFAMA) triple cation PSCs. Furthermore, the device without encapsulation can sustain 94.6% of its initial PCE after the storage at room temperature and relative humidity (RH) of 20% for 40 days. The research provides a versatile way to manipulate buried interface for achieving efficient and stable PSCs.

Constructing Additives Synergy Strategy to Doctor-Blade Efficient CH3 NH3 PbI3 Perovskite Solar Cells under a Wide Range of Humidity from 45% to 82.

Perovskite solar cells (PSCs) have emerged as one of the most promising and competitive photovoltaic technologies, and doctor-blading is a facile and robust deposition technique to efficiently fabricate PSCs in large scale, especially matching with roll-to-roll process. Herein, it demonstrates the encouraging results of one-step, antisolvent-free doctor-bladed methylammonium lead iodide (CH3 NH3 PbI3, MAPbI3 ) PSCs under a wide range of humidity from 45% to 82%. A synergy strategy of ionic-liquid methylammonium acetate (MAAc) and molecular phenylurea additives is developed to modulate the morphology and crystallization process of MAPbI3 perovskite film, leading to high-quality MAPbI3 perovskite film with large-size crystal, low defect density, and ultrasmooth surface. Impressive power conversion efficiency (PCE) of 20.34% is achieved for doctor-bladed PSCs under the humidity over 80% with a device structure of ITO/SnO2 /MAPbI3 /Spiro-OMeTAD/Ag. It is the highest PCEs for one-step solution-processed MAPbI3 PSCs without antisolvent assistance. The research provides a facile and robust large-scale deposition technique to fabricate highly efficient and stable PSCs under a wide range of humidity, even with the humidity over 80%.

Restricting the Formation of Pb-Pb Dimer via Surface Pb Site Passivation for Enhancing the Light Stability of Perovskite.

Poor light stability hinders the potential applications of perovskite optoelectronic devices. Recent experiments have demonstrated that the passivation surface via forming strong chemical bonds (SO4 -Pb, PO4 -Pb, Cl-Pb, O-Pb, and S-Pb) could effectively improve the light stability of perovskite solar cells. However, the underlying reasons are not clear. Herein, the elusive underlying mechanisms of light stability enhancement are explained in detail using first principles calculations. The small polaron model and self-trapped exciton model demonstrate that an iodine vacancy defect on the surface of perovskite could trap a free electron under light illumination, which leads to a significant rearrangement of the Pb-I lattice and creats a new chemical species, i.e., a Pb-Pb dimer bound in the typical perovskite of CH3 NH3 PbI3 . The Pb-Pb dimer distorts the Pb-I octahedral lattice and reduces the defect formation energy of the I atoms. The surface Pb site passivation can prevent the formation of the Pb-Pb dimer, thereby improving the light stability. In addition, the strong ionic bond could better stabilize the Pb site. The in-depth understanding of the light stability and the passivation mechanism in this study can promote the application of perovskite optoelectronic devices.

Iodide Vacancy Defects Clustering in Pairs Rather Than in Isolation in a Lead Iodide Perovskite: Identification, Origin, and Implications.

Iodide (I-) vacancy defects are strongly related to the stability of perovskite optoelectronic devices. The I- vacancy in lead iodide perovskites is normally considered to exist in the form of a single isolated defect. However, we determined that the I- vacancies cluster in pairs in specific ways in the typical perovskite of tetragonal CsPbI3. This I- vacancy-vacancy dimer is energetically more favorable than two isolated I- monovacancies. It breaks the symmetry of the Pb-I octahedron, resulting in lattice distortion. Its origin lies in the special lattice distortion effect caused by the electron orbital interaction of the perovskite material. Furthermore, the I- vacancy-vacancy dimer and the associated lattice distortion increase the carrier lifetime by 1.3 times compared to that of the system with two isolated I- monovacancies, but they also compromise its structural stability. This new insight into the I- vacancy defect will enhance our understanding of perovskite optoelectronic devices.

Manipulating the Migration of Iodine Ions via Reverse-Biasing for Boosting Photovoltaic Performance of Perovskite Solar Cells.

Perovskite solar cells (PSCs) are being developed rapidly and exhibit greatly potential commercialization. Herein, it is found that the device performance can be improved by manipulating the migration of iodine ions via reverse-biasing, for example, at -0.4 V for 3 min in dark. Characterizations suggest that reverse bias can increase the charge recombination resistance, improve carrier transport, and enhance built-in electric field. Iodine ions including iodine interstitials in perovskites are confirmed to migrate and accumulate at the SnO2 /perovskite interface under reverse-basing, which fill iodine vacancies at the interface and interact with SnO2 . First-principles calculations suggest that the SnO2 /perovskite interface with less iodine vacancies has a stronger interaction and higher charge transfer, leading to larger built-in electric field and improved charge transport. Iodine ions that may pass through the SnO2 /perovskite interface are also confirmed to be able to interact with Sn4+  and passivate oxygen vacancies on the surface of SnO2 . Consequently, an efficiency of 23.48% with the open-circuit voltage (Voc ) of 1.16 V is achieved for PSCs with reverse-biasing, as compared with the initial efficiency of 22.13% with a Voc  of 1.10 V. These results are of great significance to reveal the physics mechanism of PSCs under electric field.