Organic solar cells rely on carbon-based materials including polymers — as opposed to hard, inorganic materials like silicon — to capture sunlight and translate it into current. Organics are also thin, lightweight, semi-transparent, and inexpensive. While commercial silicon-based solar cells perform at about 22 percent efficiency, organics top out at around 15 percent.
One approach to fixing the problem is to find polymers or other organic semiconductors that are flexible by nature. A new approach incorporates a network of elastic additives that make the electrically active material less brittle with little to no loss of current flow. Rather than make a mesh and pour in the semiconducting polymers, sulfur-based thiol-ene reagents were mixed in. The molecules blend with the polymers and then crosslink with each other to provide flexibility. Too little thiol-ene leaves the crystalline polymers prone to cracking under stress, while too much dampens the material’s efficiency.
The researchers replaced 20 percent of the active layer with the mesh. Cells retained their efficiency and gained flexibility. The next step was to stretch the material. Pure P3HT (the active polythiophene-based layer) started cracking at about 6 percent strain. When 10 percent thiol-ene was added, the percentage increased to 14. At around 16 percent strain, cracks were observed throughout the material.
At strains higher than 30 percent, the material flexed well but became useless as a solar cell. Essentially, there was no loss in photocurrent up to about 20 percent. Damage under strain affected the material even when the strain was released.
The researchers are testing different organic photovoltaic materials while working to make them more stretchable with less additive for larger test cells.