Organic‐inorganic hybrid perovskite solar cells are susceptible to multiple influencing factors such as moisture, oxygen, heat stress, ion migration. Given the complex practical working conditions for solar cells, a fundamental question is how different failure mechanisms collaborate and substantially accelerate the device degradation. In this study, it is found that ion migration can accelerate the reaction between oxygen and methylammonium lead iodide perovskite in light conditions. This is suggested since regions with local electric fields suffer from more severe decomposition. Here it is reported that cesium ions (Cs+) incorporated in perovskite lattice, with a moderate doping concentration (e.g. 5%), can function as stabilizers to efficiently interrupt such a synergistic effect between oxygen induced degradation and ion migration while retaining the high performance of perovskite solar cells. Both experimental and theoretical results suggest that 5% Cs+ ions incorporation simultaneously suppresses the formation of reactive superoxide ions (O2−) as well as ion migration in perovskites by forming additional energy barriers. This A‐site cations engineering is also a promising strategy to circumvent the detrimental effect of oxygen molecules in FA‐based perovskites, which is important for developing high‐efficiency perovskite solar cells with enhanced stability.
Dion–Jacobson (DJ)‐type quasi‐two‐dimensional perovskites exhibit improved stabilities than their 3D counterparts but meanwhile limited charge transport properties. Knowledge to manipulate the crystal orientation and crystallinity is the primary issue for DJ perovskite with high power conversion efficiencies (PCEs). Herein, the nucleation of DJ perovskite films is divided into three stages and the formation of PbI2–N,N‐dimethylformamide (DMF)‐based solvated phase (PDS) is highlighted as the initial stage. For the first time, it is demonstrated that regulating the amount of PDS precipitation in stage I by MACl additive is the key to ensure the downward growth of DJ perovskites with out‐of‐plane orientation and high crystallinity in stage III, which is valid for DJ perovskites with different bukly organic cations including p‐phenylenediamine (PPD), p‐xylylenediamine (PXD), and propane‐1,3‐diammonium (PDA). For (PXD)(MA)2Pb3I10‐based perovskite solar cells, the PDS engineering lead to a dramtically improved PCE from 1.2% to 15.6%. Moreover, based on temperature‐dependent ionic conductivity measurement, it is confirmed that the ion migration in DJ perovskite films is efficiently suppressed, despite the possible coexisting 3D perovskite phase. The unencapsulated PXD‐based DJ perovskite devices retain over 90% efficiencies after 700 h of continuous illumination or 1500 h of storage in glove box.
Recently, the two-dimensional material Ti3C2T x MXene has attracted interest from researchers in perovskite solar cells (PSCs) with its great advantages in terms of high transmittance, high conductivity, tunable work function, and solution processability. However, the MXene-based PSC performance has still been inferior to that of the traditional TiO2- or SnO2-based counterpart up until now. Some critical issues regarding to the MXene/perovskite interface still have not been well addressed. Herein, we used the Ti3C2T x MXene as electron transport layer in PSCs via a room-temperature solution process followed by oxygen plasma treatment. Various characterization techniques were taken to establish the correlation between the surface properties and termination groups of MXene. We showed that oxygen plasma treatment could break parts of Ti–C bonds and generate abundant Ti–O bonds randomly distributed on MXene. The surface modification resulted in tunable work functions of MXene, as well as reduced trap states and improved electron transport close to the interface. In addition, the surface tension of MXene and corresponding perovskite morphology were thoroughly investigated by the contact angle and topography measurements. High-resolution XPS spectra indicated the Pb–O interactions between perovskite and MXene, which contributed to the device stability improvement.
To fabricate stable neat FAPbI3 perovskite with a pure α‐phase (pure α‐FAPbI3) is important in the field of photovoltaic commercialization because of its better bandgap than its alloying counterpart with cesium (Cs) or methylammonium (MA) cations. In this study, the first vapor deposited pure α‐FAPbI3 thin film solar cell with a power conversion efficiency (PCE) over 20% is achieved by regulating the phase transition process. It is found that under high humidity conditions, a fast phase transition between high‐purity α‐ and δ‐phase FAPbI3 can be realized. Moreover, theoretical calculations interestingly reveal a phase transition shortcut induced by the abnormal volume contraction that is ascribed to the formation of double hydrogen bonds at a certain H2O concentration. Therefore, a high‐humidity post‐treatment strategy is proposed to fabricate α‐FAPbI3 solar cells with a champion PCE of 20.19% (0.1 cm2) and 18.91% (1 cm2), which is currently the highest recorded value in vapor deposited pure α‐FAPbI3 perovskite solar cells. This study helps to redefine the effect of a water molecule on FAPbI3 solar cells. In addition, the demonstrated scaling‐up possibility provides another perspective for fabricating uniform high‐performance pure α‐FAPbI3 perovskite solar cells.
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