Printing technique gives perovskite solar cells a boost

The meniscus-assisted solution printing (MASP) technique is claimed to boost power conversion efficiencies by controlling crystal size and orientation.

The process, which uses parallel plates to create a meniscus of ink containing the metal halide perovskite precursors, could be scaled up to rapidly generate large areas of dense crystalline film on a variety of substrates, including flexible polymers. Operating parameters for the fabrication process were chosen by using a detailed kinetics study of perovskite crystals observed throughout their formation and growth cycle.

"We used a meniscus-assisted solution printing technique at low temperature to craft high quality perovskite films with much improved optoelectronic performance," said Zhiqun Lin, a professor in the School of Materials Science and Engineering at the Georgia Institute of Technology. "We began by developing a detailed understanding of crystal growth kinetics that allowed us to know how the preparative parameters should be tuned to optimise fabrication of the films."

The new technique is reported in Nature Communications.

Current perovskite fabrication techniques produce small crystalline grains whose boundaries can trap the electrons produced when photons strike the materials. Existing production techniques for preparing large-grained perovskite films typically require higher temperatures, which is not favorable for polymer materials used as substrates - which could help lower the fabrication costs and enable flexible perovskite solar cells.

Lin, research scientist Ming He and colleagues adopted an approach that uses capillary action to draw perovskite ink into a meniscus formed between two nearly parallel plates approximately 300 microns apart. The bottom plate moves continuously, allowing solvent to evaporate at the meniscus edge to form crystalline perovskite. As the crystals form, fresh ink is drawn into the meniscus.

"Because solvent evaporation triggers the transport of precursors from the inside to the outside, perovskite precursors accumulate at the edge of the meniscus and form a saturated phase," Lin said. "This saturated phase leads to the nucleation and growth of crystals. Over a large area, we see a flat and uniform film having high crystallinity and dense growth of large crystals."

To establish the optimal rate for moving the plates, the distance between plates and the temperature applied to the lower plate, the researchers studied the growth of perovskite crystals during MASP. The team observed that the crystals first grow at a quadratic rate, but slow to a linear rate.

"When the crystals run into their neighbours, that affects their growth," said He. "We found that all of the grains we studied followed similar growth dynamics and grew into a continuous film on the substrate."

The MASP process generates relatively large crystals - 20 to 80 microns in diameter - that cover the substrate surface. Having a dense structure with fewer crystals minimises the gaps that can interrupt the current flow, and reduces the number of boundaries that can trap electrons and holes and allow them to recombine.

Using films produced with MASP, the researchers have built solar cells that have power conversion efficiencies averaging 18 per cent - with some as high as 20 per cent. The cells have been tested with more than 100 hours of operation without encapsulation.