Researchers at the Georgia Institute of Technology, Atlanta, Georgia (US), have developed a new low-temperature meniscus-assisted solution printing (MASP) technique that can boost power conversion efficiencies of perovskite solar cells to nearly 20%.
“We realize the solution printing of high-quality metal halide perovskite thin films at low temperature by manipulating the solvent evaporation at the concave-shaped meniscus ink, leading to the growth of microscale perovskite crystal grains with preferred crystal orientation,” says Professor Zhiqun (Zee-Chin) Lin in the School of Materials Science and Engineering at Georgia Tech.
In more detail, he explains that the meniscus is established by confining the solution ink between a lower substrate and an upper plate by capillary force — the solvent evaporates faster at the air/ink/substrate contact line of the meniscus. This results in a convective fluid flux to replenish the evaporative loss of solvent, simultaneously transporting the perovskite solutes towards the contact line. This perovskite supersaturation phase with continually aggregated perovskite solutes results in the nucleation and growth of perovskite crystals. The convective fluid flux largely facilitates the diffusion of perovskite solutes towards the crystal grains and eventually promotes the growth of microscale perovskite crystals at a low temperature of 60 degrees Celsius.
“Notably, the perovskite films produced by this meniscus-assisted solution printing technique exhibit remarkable optoelectronic properties,” Lin says.
Controlling the crystallization of perovskite films
Solution-based processing could be a cheap, energy-efficient, high-throughput path to large-area electronic devices if it were not for the lack of control of the film morphology. To date, however, precisely controlling the film morphology in solution processing perovskite films has proven particularly challenging. Crystallization in the dynamic flow of perovskite solution has been little understood. “To this end,“ says Lin, “we demonstrated in this work that the crystallization of perovskite films can be judiciously controlled by capitalizing on the meniscus effect associated with solution printing.”
Central to this strategy is the outward convective flow triggered by the evaporation of the solvent, which facilitates the diffusion of perovskite solutes towards the crystal grains by offsetting the dragging force, eventually promoting the growth of microscale perovskite crystals at low temperature. “More significantly, we are able to uncover the perovskite crystal growth kinetics during MASP, which has never been reported before,” Lin notes.
His team discovered that the perovskite crystals grow in two stages, providing insight into the precise control over the crystal morphology and crystallinity of perovskite films for high-performance optoelectronic devices by solution printing.
Path to more efficient perovskite solar cells
Employing the meniscus effect in solution printing enables the direct crystallization of large perovskite grains with preferred crystal orientation. “In this regard, our MASP-deposited perovskite films exhibit a low charge trap density of ~5.74×1012 cm-3 and a long carrier lifetime of 1104 ± 251 ns, leading to high efficiencies in the resulting perovskite solar cells,” says Lin, adding that the high-quality large perovskite grains produced by MASP also exhibit higher resistance to light and moisture induced degradation.
Advantages of MASP
According to the expert, the big difference between his team’s meniscus-assisted solution printing approach and other thin-film coating techniques, such as doctor-blade coating, lies in that the meniscus effect acts as the major driving force for solvent evaporation and solute crystallization during MASP. In removing the solvent via high-temperature evaporation, by contrast, the ink solution is too volatile to render the good control over the crystal morphology and may cause thermal degradation and thermo-mechanical fatigue of the perovskite and electrode materials, especially for flexible electrodes such as polyethylene terephthalate (PET).
New fundamental understanding
“We successfully explored the crystal growth kinetics of the perovskite film during MASP for the first time, thereby providing a better understanding of morphology and crystallinity controls during the solution-processing deposition,” Lin says. “It is important to note that we identified the optimal window for the MASP deposition of perovskite thin films and scrutinized the effects of temperature, coating speed and meniscus geometry on the perovskite crystallization.”
Impact on the design and efficiency of future perovskite solar cells
The MASP technique developed at Georgia Tech realizes the solution printing of high-performance perovskite thin films with controllable crystal morphology at low temperature. Lin points out once more that this low-temperature printing method reduces energy consumptions and circumvents thermal degradations and thermomechanical fatigue in perovskite and electrode materials. More importantly, though, he reckons that this low-temperature strategy could eventually represent an easy way to deposit perovskite thin films on flexible polymer substrates. “We envision that the present MASP strategy may facilitate the future development and applications of perovskites for large-area, flexible optoelectronic devices at low cost,” he says.
Lin and his colleagues anticipate that the MASP deposition technique will be more complicated for producing large-area perovskite films because the crystallization rate tends to vary and become unstable at the late printing stage. “An important assumption in our model to calculate the velocity of outward convective flow induced by solvent evaporation is that the volume of loaded ink is sufficient for completing the printing process and the shape of the meniscus remains relatively unchanged with time,” says the professor. This assumption works well for printing small-area (e.g., 0.1~1 sq cm) devices. For large-area printing, the Georgia Tech team will need to maintain the meniscus shape by establishing a continuous ink supply. Lin says he knows they will need to modify their MASP setup by enabling a continuous supply of perovskite ink for large-area coating. They also want to explore uniform large-area MASP deposition of the electron-transport layer and the hole-transport layer to further improve the device performance. They aim to further improve device stability by incorporating Cs+ as an additional perovskite component and encapsulate the device to completely isolate it from contact with moisture and UV light.
The work is reported in the article "Meniscus-assisted solution printing of large-grained perovskite films for high-efficiency solar cells," published in Nature Communications.
Written by Sandra Henderson, research editor Solar Novus Today