Scientists from China's Nanjing University and Chinese Academy of Sciences have found that a change to the hole transport layer material helped reduce voltage loss in a perovskite solar cell. The discovery demonstrates a promising new way to overcome a major challenge for perovskites – particularly those used as the top layer in a tandem device.

The group of scientists noticed that a large part of the problematic voltage 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 issue.

The group experimented with 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 presented 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 reportedly 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.