Researchers at the Energy Materials and Surface Sciences Unit at the Okinawa Institute of Science and Technology Graduate University (OIST) have designed a new method to fabricate low-cost high-efficiency solar cells. Prof. Yabing Qi and his team from OIST, in collaboration with Prof. Shengzhong Liu from Shaanxi Normal University, China, developed the cells using perovskite materials.
In what Prof. Qi defines as "the golden triangle," solar cell technologies need to fulfill three conditions to be worth commercializing: their conversion rate of sunlight into electricity must be high, they must be inexpensive to produce, and they must have a long lifespan. Today, most commercial solar cells are made from crystalline silicon, which has a relatively high efficiency of around 22%. Though silicon, the raw material for these solar cells, is abundant, processing it tends to be complex and shoots up the manufacturing costs, making the finished product expensive.
Prof. Qi states that the fabrication method he and his research team have developed produces perovskite solar cells with an efficiency comparable to crystalline silicon cells, but it is potentially much cheaper than making silicon solar cells.
To make the new cells, the researchers coated transparent conductive substrates with perovskite films. They used a gas-solid reaction-based technique in which the substrate is first coated with a layer of hydrogen lead triiodide incorporated with a small amount of chlorine ions and methylamine gas - allowing them to reproducibly make large uniform panels, each consisting of multiple solar cells.
In developing the method, the scientists realized that making the perovskite layer 1 micron thick increased the working life of the solar cell significantly. "The solar cells are almost unchanged after working for 800 hours," says Dr. Zonghao Liu, a postdoctoral scholar in Prof. Qi's research unit at OIST and the first author of the study. In addition, a thicker coating not only boosted the stability of the solar cells but also facilitated the fabrication processes, thereby lowering its production costs. "The thicker absorber layer ensures good reproducibility of solar cell fabrication, which is a key advantage for mass manufacturing in the realistic industrial-scale setting," says Dr. Liu.
The big challenge Prof. Qi and his team now face is in increasing the size of these new solar cells from the 0.1 mm2 sized prototype to large commercial-sized panels that can be several feet long. To that end, Prof. Qi and team received a generous grant from OIST's Technology Development and Innovation Center, under their Proof-of-Concept Program. With that funding, the team has built a working model of their new perovskite solar modules consisting of multiple solar cells on 5cm × 5cm substrates, with an active area of 12 cm2- much bigger than their experimental prototype but smaller than what is required for commercial purposes. Although the process of up-scaling has reduced the efficiency of the cells from 20% to 15%, the researchers are optimistic that they will be able to improve the way they work in the coming years and successfully commercialize their use.