MACl-Induced Intermediate Engineering for High-Performance Mixed-Cation Perovskite Solar Cells.

Recently, mixed-cation perovskites have been extensively used for high-performance solar cells. Nevertheless, the mixed-cation perovskite based on formamidinium methylammonium lead tri-iodide (FAxMA1-xPbI3) fabricated through the existing methods often suffers from phase stability and trap density. Herein, we demonstrate a facile intermediate engineering approach to improve the quality of the mixed-cation perovskite based on FAxMA1-xPbI3. Varying concentrations of methylammonium chloride (MACl) are used to treat the FA-MA-PbI3-solvent intermediate. It is noted that MACl has a strong impact on the crystallization kinetics and charge carrier dynamics as well as the defect density of the obtained perovskite. The mixed-cation perovskite treated with 20 mg mL-1 MACl yields a large grain size, highly uniform morphology, and better crystalline stability. Subsequently, the device with an acquired high-quality mixed-cation perovskite shows a high efficiency of 20.40%, which is obviously higher than that obtained from the traditional nontreated method. Moreover, the device prepared through the developed method could retain over 85% of the initial efficiency after 860 h at room temperature.

Template-Assisted Formation of High-Quality α-Phase HC(NH2)2PbI3 Perovskite Solar Cells.

Formamidinium (FA) lead halide (α-FAPbI3) perovskites are promising materials for photovoltaic applications because of their excellent light harvesting capability (absorption edge 840 nm) and long carrier diffusion length. However, it is extremely difficult to prepare a pure α-FAPbI3 phase because of its easy transformation into a nondesirable δ-FAPbI3 phase. In the present study, a "perovskite" template (MAPbI3-FAI-PbI2-DMSO) structure is used to avoid and suppress the formation of δ-FAPbI3 phases. The perovskite structure is formed via postdeposition involving the treatment of colloidal MAI-PbI2-DMSO film with FAI before annealing. In situ X-ray diffraction in vacuum shows no detectable δ-FAPbI3 phase during the whole synthesis process when the sample is annealed from 100 to 180 °C. This method is found to reduce defects at grain boundaries and enhance the film quality as determined by means of photoluminescence mapping and Kelvin probe force microscopy. The perovskite solar cells (PSCs) fabricated by this method demonstrate a much-enhanced short-circuit current density ( J sc) of 24.99 mA cm-2 and a power conversion efficiency (PCE) of 21.24%, which is the highest efficiency reported for pure FAPbI3, with great stability under 800 h of thermal ageing and 500 h of light soaking in nitrogen.

Eliminating Charge Accumulation via Interfacial Dipole for Efficient and Stable Perovskite Solar Cells.

Elimination of interfacial charge trapping is still a challenge for promoting both efficiency and operational stability of organic-inorganic perovskite solar cells (PSCs). Herein, an effective interface dipole, trimethylamine oxide (TMAO) regarded as a connecting bridge, is inserted between the electron transport layer (ETL) and the perovskite layer to suppress charge accumulation and fabricate highly efficient and stable PSCs. As demonstrated by energy level alignment and morphology characterization, TMAO dipoles could achieve a decreased energetic barrier of electron transport and substantial padding of perovskite in the mesoporous ETL. Thus, they facilitate the charge transfer and reduce trapped charge densities as well as recombination centers at the interface between perovskite and ETL. These desirable properties improve the device efficiency to 21.77% and weaken the hysteresis index almost to 0. More importantly, the stability of the unencapsulated PSCs is remarkably enhanced. The findings provide valuable insights into the role of a dipolar molecule in boosting the performance of PSC devices.

Low-Temperature Annealed Perovskite Films: A Trade-Off between Fast and Retarded Crystallization via Solvent Engineering.

Currently, in the field of photovoltaics, researchers are working hard to produce efficient, stable, and commercially feasible devices. The prime objective behind the innovation of any photovoltaic device is to yield more energy with easy manufacture and less process cost. Perovskite solar cells (PSCs) are prominent in the field of photovoltaics, owing to its low material cost, simple fabrication process, and ideal optoelectronic properties. Despite rapid augmentation in progress of PSCs, it is still a bottleneck to produce a high-quality perovskite layer at low temperatures in a short time. Herein, a facile solvent engineering technique is used to produce a high-quality perovskite layer at 50 °C in just 30 min. We employed solvent coordination strength to form the intermediate state as well as their sensitive behavior against antisolvent to establish a trade-off between fast and retarded crystallization. Dimethylsulphoxide (DMSO), a traditional co-solvent is used as an additive instead of co-solvent; in contrast, mixed 1-methyl-2-pyrrolidinone (NMP) and dimethylacetamide are employed as principal solvents for perovskite precursors. Different volume ratios of DMSO as a fraction of NMP are added to examine the evolution of the perovskite layer at low temperatures. It is noted that the mixed solvent with 30% DMSO shows a pin-hole free, uniform, and compact layer with a strong absorption spectrum. Promisingly, the corresponding device with 30% DMSO shows a high efficiency of 18.19%, which is even comparable to traditionally high-temperature annealed PSCs. These findings may provide a way to produce low-temperature annealed, high-quality perovskite films and subsequently facilitate the production of cost-effective and efficient devices.