Songyuan Dai

Temperature-assisted rapid nucleation: a facile method to optimize the film morphology for perovskite solar cells

The nucleation stage has an important influence on the lead halide perovskite film morphology, and therefore the solar cell performance. Here, we introduce a facile temperature-assisted rapid nucleation (TRN) method to improve the film crystallization process and the film morphology. By employing low temperature anti-solvents (diethyl ether, chlorobenzene and toluene), we stimulate homogeneous nuclei growth, resulting in a highly dense perovskite thin layer. The TRN method prepared (FAPbI3)0.85(MAPbBr3)0.15 devices exhibit a power conversion efficiency up to 19.2% and maintain over 85% of the original efficiency after 40 days of storage at room temperature under a relative humidity of 10–40%.

Facile fabrication of perovskite layers with large grains through a solvent exchange approach

Large-grained perovskite layers with fewer defects are beneficial to minimize carrier recombination at grain boundaries, thus boosting cell performance. Here, we present a facile solvent exchange strategy (SES) to yield large-grained perovskite films with high reproducibility. Upon employing a stoichiometric PbI2 : MAI : DMSO (3 : 2 : 2) precursor solution, we reduced the number of colloidal intermediates (PbI42−) which is correlated with the nucleation of seeds, resulting in a perovskite thin layer with large grain sizes up to 2 μm. The SES prepared CH3NH3PbI3 devices show significantly reduced grain boundaries corresponding to the decrease of recombination centers, which dramatically boosted the efficiency up to 17.2%.

High-Efficiency and UV-Stable Planar Perovskite Solar Cells Using a Low-Temperature, Solution-Processed Electron-Transport Layer

Efficient metal-halide perovskite solar cells (PSCs) with a regular structure typically use high-temperature-processed TiO2 electron-transport layers (ETLs), which suffer from high electron recombination and inherent UV instability. Herein, we present low-temperature solution-processed lithium bis(trifluoromethanesulfonyl)imide (Li-TFSI)-doped C60 (Li-C60 ) ETLs for high-efficiency and UV-stable planar PSCs with an n-i-p architecture. We found that simple Li-TFSI doping ensured a better energy match between the active layer and the cathode, considerably enhanced the electron mobility of the ETL, and even improved CH3 NH3 PbI3 crystallization finally to increase the power conversion efficiency (PCE) from 15.3 to 17.8 % with a minor hysteresis effect. Moreover, it was demonstrated that replacing TiO2 with Li-C60 resulted in PSCs that were much more stable under UV light under an air atmosphere with almost no degradation after 3000 h under a nitrogen atmosphere.

Enhanced morphology and stability of high-performance perovskite solar cells with ultra-smooth surface and high fill factor via crystal growth engineering

Since solvent engineering methods were applied to the treatment of perovskite films, the performance of perovskite solar cells (PSCs) has shown rapid growth and remarkable achievements have been made. Here we report a highly reproducible method for controlling perovskite crystal growth by a spraying anti-solvent process, which is quite different from conventional dripping methods. The results showed that the change of the method by which the anti-solvent is used has a significant impact on the morphology and formation of the perovskite. It has a high probability to form a mirror-like surface without wave-circle or ring defects, and to obtain a longer carrier lifetime than crystals formed by by dripping. After optimizing the spraying operating conditions, the optimal device based on FA0.81MA0.15Cs0.025PbI2.5Br0.45 obtained a PCE of 19.21%. Particularly, this method exhibited good reproducibility and a high fill factor due to the reduced crystal defects in the film. The champion cell obtained a fill factor as high as 80.84%, measured at AM 1.5G, 100 mW cm−2. It is expected that these findings can be beneficial for the future integrated applications of these perovskites.

Designing function-oriented artificial nanomaterials and membranes via electrospinning and electrospraying techniques

The sister technologies, electrospinning and electrospraying provide a facile and universal synthesis method for the continuous preparation of nanostructured materials. Through adjusting the synthesis parameters, rich electrospun and electrosprayed nanomaterials, scaffolds, membranes with tunable composition (inorganic, polymeric, hybrid, etc.), shape (sphere, films, scaffold, etc.), morphology and inner structure (solid, hollow, core-shell, co-axial, etc.) can be selectively elaborated. This review provides an overview of the design of functional nanostructured materials, porous scaffolds and membranes by electrospinning and electrospraying techniques. Key experimental parameters and synthesis strategy are emphasized to reveal the synthesis-component-structure-property relationship and eventually realize the targeted functions through predictable synthesis. Potential applications in tissue engineering, medicine, membrane filtration and lithium battery are highlighted.

Adjusting the Introduction of Cations for Highly Efficient and Stable Perovskite Solar Cells Based on (FAPbI3)0.9(FAPbBr3)0.1

Although the power conversion efficiency (PCE) of perovskite solar cells (PSCs) has increased to 22.7 %, the instability when exposed to moisture and heat has hindered their further practical development. In this study, to gain highly efficient and stable perovskite components, methylammonium (MA), Cs, and Rb cations are introduced into a (FAPbI3 )0.9 (FAPbBr3 )0.1 (FA=formamidine) film, which is rarely used because of its poor photovoltaic performance. The effects of different contents of MA, Cs, or Rb cations on the performance of (FAPbI3 )0.9 (FAPbBr3 )0.1 films and devices are systematically studied. The results show that the devices with Cs cations exhibit markedly improved photovoltaic performance and stability, attributed to the clearly enhanced quality of films and their intrinsic stability. The (FAPbI3 )0.9 (FAPbBr3 )0.1 devices with 10 % Cs show a PCE as high as 19.94 %. More importantly, the unsealed devices retain about 80 % and 90 % of the initial PCE at 85 °C after 260 h and under 45±5 % relative humidity (RH) after 1440 h, respectively, which are better than that with 15 % MA and 5 % Rb under the same conditions. This indicates that a highly efficient and stable perovskite component has been achieved, and PSCs based on this component are expected to promote their further development.

Efficient Solar Cells with Enhanced Humidity and Heat Stability Based on Benzylammonium-caesium-formamidinium Mixed-Dimensional Perovskites

Perovskite solar cells (PSCs) exhibit remarkable photovoltaic performance with a power conversion efficiency (PCE) over 22%, but they exhibit instability in moist environments and at high temperatures. Compared to 3D perovskites, two-dimensional (2D) layered perovskites display excellent environmental stability but relatively poor photovoltaic performance. Here, we combined 2D/3D perovskites and simultaneously introduced the cesium cation (Cs+) to fabricate benzylammonium–caesium–formamidinium mixed-dimensional (MD) perovskite (BE/FA/Cs MD perovskite) solar cells. The BE/FA/Cs MD perovskite device with an optimal benzylammonium content exhibits a PCE as high as 19.24%. The improved PCE of 19.24% (BE/FA/Cs MD, x = 0.05) is attributed to great crystal orientation, outstanding surface quality, superior optical properties and enhanced charge transfer. More importantly, the BE/FA/Cs MD perovskite devices display superior humidity and heat stability. When subjected to 50% relative humidity (RH) for 1600 h and 85 °C for 240 h in the dark, the BE/FA/Cs MD (x = 0.05) devices without encapsulation retain 85% and 83% of their initial PCE, respectively. These results provide us with an important method to obtain highly efficient MD PSCs with long-term stability as a next-generation photovoltaic energy source.

High-Performance Mixed-Dimensional Perovskite Solar Cells with Enhanced Stability against Humidity, Heat and UV Light

Despite three-dimensional (3D) perovskite solar cells (PSCs) having a high power conversion efficiency (PCE) beyond 22%, they are unstable under humidity, oxygen and ultraviolet light. Two-dimensional (2D) perovskites exhibit high stability but have poor photovoltaic performance. To simultaneously enhance the PCE and stability of PSCs, we designed new mixed-dimensional (MD) PSCs by introducing HOCH2CH2NH3I (EAI) into (FAPbI3)0.85(MAPbBr3)0.15 3D perovskite. The MD devices made with [(EA)2PbI4]x[(FAPbI3)0.85(MAPbBr3)0.15]1−x exhibit outstanding photovoltaic performance with an improved PCE of 18.79% due to the outstanding crystal structure, superior surface morphology, enhanced charge transport and reduced carrier recombination. More importantly, the MD devices display markedly improved stability against humidity, heat and UV light. After exposing to about 50% relative humidity (RH) for over 1700 hours (h), the unsealed devices retain about 85% of the initial PCE. After exposing to air at 85 °C for 220 h and exposing to continuous UV irradiation for 13 h, all the unsealed devices retain PCEs of about 60% of the initial value. These results demonstrate that designing a novel MD perovskite is a significant strategy for improving both photovoltaic performance and stability of PSCs.

Acquiring High-Performance and Stable Mixed-Dimensional Perovskite Solar Cells by Using a Transition-Metal-Substituted Pb Precursor

In recent years, as the most promising photovoltaic technology, lead halide perovskite solar cells (PSCs) have become a research hotspot owing to their super-high power conversion efficiency (PCE) and simple preparation method. However, the toxicity of lead and instability under high humidity and temperature greatly affects their further development. In this study, we investigated three kinds of non-toxic transition metal cations (Zn2+ , Mn2+ , and Ni2+ ) at 3 % to partially substitute lead cations and form mixed-dimensional (MD) perovskites. Among these transition metals, Zn2+ is the most suitable to replace Pb2+ and Zn-MD perovskite device exhibits the best PCE of 18.35 %. In addition, we further studied the humidity and heat stability of methylammonium lead iodide (MA), Zn-MD, Mn-MD, and Ni-MD perovskite devices. Upon exposure to about 50 % relativity humidity (RH) for 800 h, the MA and three MD devices, respectively, maintain 30 %, 81 %, 80 %, and 74 % of their original PCE. After aging at 60 °C for 100 h, the PCEs of the four PSCs retain 40 %, 84 %, 85 %, and 76 % of their starting values, respectively. The results show that the three MD perovskite devices have high humidity and heat stability and using transition-metal-substituted Pb can gain long-term stability and reduced toxicity of PSCs.