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Though tandem solar cells featuring at least one perovskite are well on their way to commercial production, there are still a few challenges holding the technology back from its full potential.
One of these concerns the wide bandgap required for a top cell, which often comes with large open-circuit voltage losses that limit the cell performance. A group of scientists led by Nanjing University in China noted that a large part of this loss occurs at the interface between the active perovskite and the hole transport layer (HTL) that helps to carry a charge out of the device, and decided to experiment with alternate materials to try and limit this.
The group settled on a cross-linked organic small molecule it calls VNPB. Using this material to replace the more common poly(triaryl amine) (PTAA) hole transport layer, it was able to demonstrate a boost to the open-circuit voltage of almost 50 millivolts, for cells with a bandgap between 1.6 and 1.8 electron volts. “Compared with the control device using PTAA polymeric HTL, the perovskite films deposited on VNPB have larger grain size and better crystallinity,” explained Nanjing University professor Hairen Tan. “VNPB enables faster charge extraction and reduces defect density at the HTL/perovskite interface.”
Calculations made by the group show that VNPB allows for closer contact between the two layers, and decreases the number and density of defects at the interface between the two layers. The devices fabricated with VNPB achieved 24.9% efficiency as part of an all-perovskite tandem, and 25.4% in a perovskite-silicon device. “The results demonstrate that cross-linkable small molecules are promising for high-efficiency and cost-effective perovskite tandem photovoltaic devices,” the group concluded.
The hole transport layer material and devices fabricated using it are described in the paper Cross-linked hole transport layers for high-efficiency perovskite tandem solar cells, published in Science China Chemistry.
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Source: pv magazine