Engineers Turn OPVs Upside Down

October
2015

The advent of organic photovoltaics (OPVs) brought with it the potential for low-cost, flexible solar power, with applications ranging from rollable electronic displays to building façades. That promise eventually faded as the efficiency of the bendable solar cells was insufficient for commercial use.

Engineers at Purdue Univ. have come up with a new theory to explain the low efficiency of OPV devices that could enable the development of OPVs with efficiencies in line with their rigid silicon-based cousins.

The first OPV devices contained a single organic polymer semiconducting layer sandwiched between two electrodes. The polymeric semiconductor absorbs the sunlight hitting the cell, generating excitons (electron-hole pairs), which subsequently separate into electrons and holes. An electric field created by the difference in work function of the two electrodes keeps the electrons and holes separated until the electrons can exit the cell to provide the user with electricity. At the time, scientists attributed the low efficiency of those cells to inadequate exciton separation (free-charge generation), which caused the electrons and holes to recombine before the electrons could exit the cell.

“Because the electron is negatively charged and the hole is positively charged, they like each other so much that they orbit around each other,” says Biswajit Ray, a doctoral student in electrical and computer engineering at Purdue. “You have to keep these two separated or they will recombine and you will not generate...

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