Xiaobing Cao

Enhanced performance of perovskite solar cells by modulating the Lewis acid–base reaction

The Lewis acid-base reaction between PbI2 and solvent molecules is popular in fabricating PbI2 films by a two-step method for making perovskite solar cells. Here, we control the microstructure of PbI2 films through modulating the Lewis acid-base reaction by adding a small amount of N-methyl pyrrolidone into PbI2/DMF solution. PbI2 films with excellent crystallinity and full coverage are fabricated by spin-coating the mixed solution on the substrate, which leads to high quality perovskite layers with low recombination rate and high efficiency for carrier transfer. As a result, the power conversion efficiency of the best perovskite solar cells increases from 13.3% to 17.5%.

High quality perovskite films fabricated from Lewis acid–base adduct through molecular exchange

We describe a facile method to fabricate high quality CH3NH3PbI3 films with smooth surface and without residual PbI2 through molecular exchange, rather than ionic intercalation. It forms a Lewis acid–base adduct of PbI2·xDMF when PbI2 precipitates from DMF solution. The lattice of PbI2 is expanded more than 30% due to the intercalation of DMF. With the lattice expansion, CH3NH3I (MAI) diffuses into the PbI2·xDMF lattice easily, and the PbI2·xDMF converts completely to CH3NH3PI3 by molecular exchange between DMF and MAI. The lattice volume changes little during the molecular exchange in PbI2·xDMF. Thus, it is easy to fabricate high quality perovskite films from the Lewis adduct of PbI2·xDMF. The perovskite solar cells fabricated from the Lewis adduct exhibit higher photovoltaic performance than those from the PbI2 films. This work reveals the important role of common solvent in controlling the quality of perovskite films.

Fabrication of high quality perovskite films by modulating the Pb–O bonds in Lewis acid–base adducts

High efficiency perovskite solar cells can be made through the Lewis adduct approach. The interaction between PbI2 and the Lewis base is a key factor to make high quality perovskite films. Here, we combine a two-step method with the Lewis adduct approach to fabricate high quality organometallic perovskite films by tuning the strength of the Pb–O bond in Lewis acid–base adducts via adding different Lewis base additives. By optimizing the strength of the Pb–O bond, smooth CH3NH3PbI3 films with large perovskite grains are prepared. The high quality CH3NH3PbI3 films lead to a low recombination rate, long carrier life time and highly efficient charge transfer process. As a result, the power conversion efficiency of the best perovskite solar cell increases from 12.79% to 17.26%.

Control of the morphology of PbI2 films for efficient perovskite solar cells by strong Lewis base additives

A two-step method is widely used to fabricate highly efficient perovskite solar cells. In a typical two-step method, CH3NH3PbI3 perovskites are fabricated from PbI2 films through an intercalation reaction between PbI2 and CH3NH3I, which usually have a rough surface and residual PbI2 due to a compact PbI2 structure. Here, we introduce some strong Lewis bases into PbI2/DMF solutions in the first step, which is helpful in controlling the morphology of PbI2 films with a mesoporous structure. The mesopores in the PbI2 films provide not only space for accommodating the volume expansion during the reaction between PbI2 and CH3NH3I, but also channels for CH3NH3I solution to diffuse into the PbI2 films, which helps eliminate the residual PbI2 and the dissolution and recrystallization process. As a result, smooth perovskite films without residual PbI2 are easily obtained. The power conversion efficiency, stability and reproducibility of the perovskite solar cells fabricated from the optimized mesoporous PbI2 films improved simultaneously due to the improvement in perovskite films. This work reveals the key roles of solvents (boiling point, Lewis basicity) in controlling the microstructure of the PbI2 films.

In Situ Observation of Crystallization of Methylammonium Lead Iodide Perovskite from Microdroplets

It is of great importance to investigate the crystallization of organometallic perovskite from solution for enhancing performance of perovskite solar cells. Here, this study develops a facile method for in situ observation of crystallization and growth of the methylammonium lead iodide (MAPbI3 ) perovskite from microdroplets ejected by an alternating viscous and inertial force jetting method. It is found that there are two crystallization modes when MAPbI3 grows from the CH3 NH3 I (MAI)/PbI2 /N,N-dimethylformamide (DMF) solution: needle precursors and granular perovskites. Generally, needle Lewis adduct of MAPbI3 ·DMF tends to nucleate and grow from the solution due to low solubility of PbI2 . The growth of MAPbI3 ·DMF depends on both the concentration of MAI and temperature. It tends to form large perovskite domains on substrates at high temperature. The MAPbI3 ·DMF coverts to nanocrystalline perovskite due to lattice shrinkage when DMF molecules escape from the Lewis adduct. Granular perovskite can also directly nucleate from the solution at high concentration of MAI due to compositional segregation.

Enhanced performance of perovskite solar cells by strengthening a self-embedded solvent annealing effect in perovskite precursor films

The solvent embedded in the intermediate phase is widely observed in the fabrication of perovskite films. The perovskite precursor films obtained from Lewis adducts through molecular exchange contain some residue solvent. It has an intrinsic solvent annealing effect during the annealing process. Here, we pre-deposit a protective layer on the perovskite precursor films to retard the escape of solvent during the annealing process. The restricted solvent strengthens the solvent annealing effect during the formation of perovskite films. As a result, the perovskite quality, including grain size and crystallization, is improved significantly, which leads to efficient charge transportation, low recombination rate, and enhancement of the photovoltaic performance of the corresponding perovskite solar cells.

Elucidating the Key Role of a Lewis Base Solvent in the Formation of Perovskite Films Fabricated from the Lewis Adduct Approach

High-quality perovskite films can be fabricated from Lewis acid-base adducts through molecule exchange. Substantial work is needed to fully understand the formation mechanism of the perovskite films, which helps to further improve their quality. Here, we study the formation of CH3NH3PbI3 perovskite films by introducing some dimethylacetamide into the PbI2/N,N-dimethylformamide solution. We reveal that there are three key processes during the formation of perovskite films through the Lewis acid-base adduct approach: molecule intercalation of solvent into the PbI2 lattice, molecule exchange between the solvent and CH3NH3I, and dissolution-recrystallization of the perovskite grains during annealing. The Lewis base solvents play multiple functions in the above processes. The properties of the solvent, including Lewis basicity and boiling point, play key roles in forming smooth perovskite films with large grains. We also provide some rules for choosing Lewis base additives to prepare high-quality perovskite films through the Lewis adduct approach.

High annealing temperature induced rapid grain coarsening for efficient perovskite solar cells

Thermal annealing plays multiple roles in fabricating high quality perovskite films. Generally, it might result in large perovskite grains by elevating annealing temperature, but might also lead to decomposition of perovskite. Here, we study the effects of annealing temperature on the coarsening of perovskite grains in a temperature range from 100 to 250 °C, and find that the coarsening rate of the perovskite grain increase significantly with the annealing temperature. Compared with the perovskite films annealed at 100 °C, high quality perovskite films with large columnar grains are obtained by annealing perovskite precursor films at 250 °C for only 10 s. As a result, the power conversion efficiency of best solar cell increased from 12.35% to 16.35% due to its low recombination rate and high efficient charge transportation in solar cells.

Crystallization of CH3NH3PbI3−xBrx perovskite from micro-droplets of lead acetate precursor solution

The physical properties of organic–inorganic hybrid perovskite materials are greatly influenced by their composition, crystallinity, and morphology. Here, we investigate the growth of CH3NH3PbI3−xBrx perovskite from micro-droplets of a methylammonium iodide/lead acetate/dimethylformamide precursor solution added with different concentrations of PbBr2, which helps to control the morphology and crystallinity of the perovskite film. We find that both perovskite and its intermediate containing an acetate group growing from the micro-droplets. By adding some PbBr2, there is a tendency to form cubic perovskite crystals due to partial substitution of iodine with bromide. There is composition segregation when the crystals grow freely from the droplets. Smooth and uniform CH3NH3PbI3−xBrx perovskite films with a tunable band gap are thus fabricated from the precursor solution added with PbBr2.