Researchers led by Distinguished Professor Seok Sang-il of the Department of Energy and Chemical Engineering at the Ulsan National Institute of Science and Technology (UNIST) have identified the root cause of a defect that had been holding back the efficiency of inverted-structure perovskite solar cells, and used the finding to raise cell efficiency to 26.3% - matching what's typically achieved in conventional-structure cells.
The inverted structure in particular has low light loss to the lower layer, which is why it's mainly used as the top cell in tandem solar cells combined with silicon. However, applying the high-efficiency material formulation developed for conventional-structure cells directly to the inverted structure had caused efficiency to plunge to around 18%. Using advanced analysis methods at the Pohang Accelerator Laboratory, the team traced the cause: methylammonium chloride (MACl), an additive that helps crystal growth in the conventional structure, instead induces defects when used in the inverted structure. On the hydrophobic organic material that forms the bottom layer of the inverted structure, the intermediate substance created by MACl escapes belatedly. During this process, gaps and cracks form at the buried interface - the bottom of the perovskite layer - which act as points where charge carriers are lost, reducing efficiency.
To address this, the team drastically reduced the MACl additive from the previous 30 mol% to 10 mol%, and instead added 2 mol% of lead chloride, which has stronger bonding strength. Lead chloride keeps chlorine near the bottom of the film, evenly controlling the timing and location where crystals form. Applying the new formulation raised the inverted cell's maximum efficiency to 26.3%, with a fill factor of 86.8%. The cells also proved durable, retaining 97.4% of their initial efficiency after about 10,000 hours of storage at room temperature.
Seok Sang-il said: "We found the root cause of why the material formulation used for conventional structures shows reduced performance in inverted structures, and we presented a universal design direction that can improve efficiency." He added: "We will lead the development of high-efficiency top cells for perovskite-silicon tandem solar cells."
