Organic photovoltaics (OPV) is an emerging area of scientific enquiry which is attracting increasing attention in the industry, as it is more sustainable and uses cheaper materials than silicon-based solar technology. Now an international collaboration between the Max Planck Institute for Polymer Research (MPI-P) and the King Abdullah University of Science and Technology (KAUST) in Saudi Arabia has led to invaluable insights into what exactly influences the efficiency of these cells.
In an OPV, light particles (photons) transfer their energy to electrons. The electrons become “excited” and leave behind positively charged “holes”. These electron hole-pairs (excitons) must then be separated by two semiconducting materials (a “donor” and an “acceptor”), which compel the electron to find another hole, thus creating a continuous movement of electrons which creates current.
The efficiency of an OPV has been put down to a number of factors: “electron affinity” and “ionization energy”, but is also influenced by colour and transparency. What the research team discovered is that the main determinant of efficiency is the difference between the ionization energy of the two semiconductor materials.
After the team at KAUST conducted a series of optical spectroscopy experiments and then MPI-P performed numerous data simulations, the exact design rules for molecular dyes have been determined in order to maximise a solar cell’s efficiency.
"In the future, for example, it would be conceivable to produce transparent solar cells that only absorb light outside the range visible to humans – but then with the maximum efficiency in this range," says Denis Andrienko from MPI-P, co-author of the study published in the journal . With the new breed of OPV materials, the entire façade of a building could be turned into an active light-absorbing surface.
The authors hope that 20 percent solar efficiency will be reached, which is the threshold target for OPVs to become commercially viable.