Printing high efficiency solar cells

Editorial

Rebecca Pool

Tuesday, July 11, 2017 - 14:45
Image: Large perovskite grain sizes boost solar cell efficiency. [Ming He, Georgia Tech]
 
Researchers have unveiled a stunning image of perovskite crystal grains following the development of a new printing method to fabricate high efficiency perovskite solar cells.
 
So-called meniscus-assisted solution printing delivers high quality perovskite films with a large crystal structure that can be used to make solar cells with a power conversion of nearly 20%.
 
“We used a meniscus-assisted solution printing technique at low temperature to craft high quality perovskite films with much improved optoelectronic performance,” highlights Professor Zhiqun Lin from 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,” he adds.
 
Research Scientist Ming He (left) and Professor Zhiqun Lin are shown in Lin’s laboratory in the School of Materials Science and Engineering at the Georgia Institute of Technology. [Rob Felt, Georgia Tech]
 
Perovskites offer an attractive alternative to traditional materials for capturing electricity from light, but existing fabrication techniques typically produce small crystalline grains with many grain boundaries that can trap the electrons produced when photons strike the materials.
 
Existing production techniques for preparing large-grained perovskite films typically require higher temperatures, which are not favourable for the polymer substrates needed to lower the fabrication costs.
 
Given this, Lin and colleagues developed a new approach that relies on 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. And 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,” explains Lin. “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 imaged the growth of perovskite crystals during MASP using optical microscopy.
 
Samples produced by the meniscus-assisted solution printing technique are studied using optical microscopy [Rob Felt, Georgia Tech].
 
 
Videos revealed that the crystals first grow at a quadratic rate, but slow to a linear rate when they impinge on neighbouring grains.
 
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.
 
So far, the researchers have produced centimetre-scale samples, but believe the process could be scaled up and applied to flexible substrates, potentially facilitating roll-to-roll continuous processing of the perovskite materials.
 
Research is published in Nature Communications.
 
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