South Korean researchers have uncovered why simply reversing the stacking order of materials causes a dramatic drop in power generation efficiency, even when using the same high-performance solar cell ingredients. The reason lies in how light-absorbing crystals grow differently depending on the bottom substrate material. The research team successfully adjusted the material formulation to match the bottom layer, raising efficiency to 26.3%.
The Ulsan National Institute of Science and Technology (UNIST) announced on August 5 that a team led by Distinguished Professor Sang Il Seok of the Department of Energy and Chemical Engineering had identified why high-efficiency perovskite solar cell materials underperform when applied to cells with a different architecture, and developed a new formulation to solve the problem. The research was published online in the energy journal Joule on July 28.
Perovskite solar cells are built by stacking a light-absorbing perovskite layer and functional materials that transport electricity. Because they are made by coating and solidifying solutions, they can be thinner and lighter than conventional silicon solar cells. Depending on the order in which functional materials are deposited, they are classified as either “regular structure” or “inverted structure.”
The problem was that a material formulation achieving over 26% efficiency in a regular structure delivered an average efficiency of just 18%—and a maximum of only 21%—when applied directly to an inverted structure. The same ingredients were used, yet performance collapsed simply because the stacking sequence changed.
The research team identified the root cause using advanced analytical techniques at the Pohang Accelerator Laboratory. The key lay in the substrate material beneath the perovskite layer. In a regular structure, the bottom substrate allows the precursor solution to spread easily, whereas the hydrophobic organic film used as the bottom layer in an inverted structure repels the solution. As a result, methylammonium chloride (MACl)—an additive that aids crystal growth—could not escape in time and lingered near the bottom in the inverted configuration. MACl, which promotes crystal growth in regular structures, instead caused defects in inverted ones.
This process generated voids and cracks at the buried interface—the bottom of the perovskite layer. These empty spaces acted as sites where charge carriers, the electrical particles, were lost, dragging down efficiency.
To solve the problem, the team devised a method that drastically reduced the MACl additive from the conventional 30 mol% to 10 mol%, while adding 2 mol% of lead chloride (PbCl₂), which has strong binding properties. The principle is that lead chloride keeps chlorine near the bottom region, uniformly controlling the timing and location of crystal formation.
When the new formulation was applied, the voids and cracks at the bottom disappeared. Maximum efficiency rose to 26.3%, comparable to regular structures, and the fill factor (FF)—which represents the ratio of actual extractable electricity—reached 86.8%. Cells that underwent encapsulation to block external air retained 97.4% of their initial efficiency after approximately 10,000 hours of storage at room temperature, confirming excellent durability.
Inverted perovskite cells are primarily used as the top cell in tandem solar cells, where a perovskite cell is placed atop a silicon solar cell. They are considered a key technology for boosting tandem cell efficiency because they minimize the loss of light and electricity transmitted to the bottom layer.
Distinguished Professor Sang Il Seok said, “We identified the fundamental reason why regular-structure formulations underperform in inverted structures. We will present a universal design direction for high-efficiency inverted solar cells and lead the development of high-efficiency top cells for perovskite-silicon tandem solar cells.” This research was conducted with support from South Korea’s National Research Foundation.
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