A new multi-university study focusing on the fundamental science of charge separation has resulted in a greatly improved understanding of the subject — an improved understanding that should lead to more-efficient organic solar cells in the near future, according to those involved.
The new work has resulted in the creation of a number of “design rules” for the technology that should lead to notable improvements in efficiency. The new research was performed by Penn State electrical engineer Noel Giebink, lead author and undergraduate Bethany Bernardo, and colleagues at IMEC in Belgium, Argonne National Lab, Northwestern, and Princeton.
Top-of-the-line organic solar cells currently possess a top efficiency of around 10% in the laboratory, considerably less than conventional forms of solar. One of the main limitations to higher efficiencies in organic solar cells is in the ability to separate “the strongly bound pairs made up of a negatively charged electron and a positively charged hole that result from light absorption, collectively referred to as an exciton.” These two components, the electron and the hole, need to be kept separate in order to create a current. This is where the new research comes in.
Penn State continues:
The way this is done is by creating a heterojunction, which is two different organic semiconductors next to each other, one of which likes to give up an electron and the other which accepts the electron, thereby splitting the original exciton into an electron and hole residing on nearby molecules. A long-standing question in the field, however, is how the nearby electron and hole — still strongly attracted to each other at this stage — manage to separate completely in order to generate current with the efficiency observed in most solar cells.
Over the past few years, a new perspective has proposed that the high separation efficiency relies on a quantum effect — the electron or hole can exist in a wavelike state spread out over several nearby molecules at the same time. When the wave function of one of the carriers collapses at a location far enough away from its partner, the charges can separate more easily. Giebink and colleagues’ work provide compelling new evidence to support this interpretation and identify nanocrystallinity of the common acceptor material made of C60 molecules (also known as fullerenes or buckyballs) as the key that allows this delocalization effect to take place.
“This local crystalline order appears to be critical to efficient photocurrent generation in organic solar cells,” states Giebink. “A common view in the community is that it takes a bunch of excess energy to break apart the exciton, which meant that there had to be a large energy level difference between the donor and acceptor materials. But that big energy offset reduces the voltage of the solar cell. Our work dispels this perceived tradeoff in light of the impact that wavefunction delocalization and local crystallinity have on the charge separation process. This result should help people design new molecules and optimize donor and acceptor morphologies that help increase solar cell voltage without sacrificing current.”
The new findings were published in the journal Nature Communications.
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