The organic solar cell technology promises low-cost manufacturing, abundant raw materials, and lightweight, flexible substrates. These benefits address the limitations of current photovoltaic cells on the market. However, the efficiency of this cheaper and more sustainable alternative to silicon-based solar cells remains a big challenge. At 11 percent efficiency, they need more ways to catch up on silicon-based solar cells’ efficiency.
A new research group called the Center for Computational Study of Excited-State Phenomena in Energy Materials, established in 2016 at the Berkeley Lab, has opened a new direction to improve organic solar cell’s efficiency. “We actually discovered a new mechanism that allows us to try to design better materials,” says C2SEPEM Director Steven G. Louie.
What Are Organic Solar Cells and How Do They Work?
Organic solar cells or plastic solar cells are made up of flexible polymeric materials instead of the usual silicon wafers. By capturing photons in its semiconducting polymeric absorber, an organic solar cell allows the conversion of light energy directly into electricity.
The mechanism of electricity generation can be described in four phases. (1) Light absorption: photons are captured in the donor layer, which is the polymeric absorbing material. The light energy excites the photoactive material, allowing the formation of excitons within the donor layer. Excitons are electron-hole pairs that are bound or localized within the material. (2) Exciton diffusion: concentration gradient of excitons between the donor and acceptor layers makes the excitons diffuse into the donor-acceptor interface. (3) Charge separation: the ‘hole’ or positive part of each exciton transfers to the acceptor layer, while the ‘electron’ or the negative part goes to the donor. (4) Charge extraction: These positive and negative carriers then move toward the cathode and anode, respectively.
Increasing the Efficiency thru Singlet Fission
Two years ago, a group of chemists from the University of California was able to improve the efficiency of organic solar cells through a process called the ‘singlet fission.’ In this process, one photon can generate two excitons instead of only one.
The singlet fission is illustrated in the above image, where the red object represents the excited singlet state, which features an electron-hole pair. The singlet splits into a pair of triplet states, represented by blue objects.
One of the chemists explains the potential of the process: “If a triplet exciton has half the energy of a singlet, then it is possible for one singlet exciton, generated by one photon, to split into two triplet excitons. Thus, you could have a 200% yield of excitons—and hopefully, electrons—per absorbed photon.”
The study entitled, “Origins of Singlet Fission in Solid Pentacene from an ab initio Green’s Function Approach,” was published last December in the journal Physical Review Letters. It differs from other studies such that it tries to understand the singlet fission phenomenon through a holistic approach, rather than examining just the localized reactions occurring within a few molecules.
As co-lead author, Felipe H. da Jornada, explains, “It’s like trying to explain the ocean by either looking at it molecule by molecule or looking at a whole wave…Our approach directly captures the whole crystal.”
Using pentacene as the semiconducting material, the research team discovered that in order for a material to produce singlet fission efficiently, its structure must have symmetry and dense packing of molecules in each symmetrical unit.
“The efficiency of the singlet fission process appears to rely heavily on the number of molecules packed within each repeating pattern or “motif” in the crystal, and on a particular type of symmetry that in which there is a 180-degree rotation and mirroring of these motifs. This relationship between symmetry and efficiency, the researchers found, allows them to make powerful predictions on the efficiency of the overall fission.”