Organic Photovoltaics Research

DOE funds research and development projects related to organic photovoltaics (OPV) due to the unique benefits it offers. Below are a list of the projects, summary of the benefits, and discussion on the production and manufacturing of this solar technology.

Background

Organic photovoltaic (OPV) solar cells aim to provide an Earth-abundant and low-energy-production photovoltaic (PV) solution. This technology also has the theoretical potential to provide less expensive energy than first- and second-generation solar technologies. Because various absorbers can be used to create colored or transparent OPV devices, this technology is particularly appealing to the building-integrated PV market. Although OPV cells are fast approaching the 10% efficiency mark, efficiency limitations as well as long-term reliability remain significant barriers.
Unlike most inorganic solar cells, OPV cells use single-molecule absorbers. Therefore, an exciton is generated across a single molecule, instead of between bulk electronic states. These molecular absorbers are used in conjunction with an electron acceptor, such as a fullerene, which has molecular orbital energy states that facilitate electron transfer. Upon absorbing a photon, the resulting exciton migrates to the interface between the absorber material and the electron acceptor material. At the interface, the energetic mismatch of the molecular orbitals provides sufficient driving force to split the exciton and create free charge carriers (an electron and a hole).

Research Directions

The low efficiencies of OPV cells are related to their small exciton diffusion lengths (~ 10 nm) and low carrier mobilities (>5cm²/V^.s). These two characteristics ultimately result in the use of thin active layers that affect overall device performance. Furthermore, the operational lifetime of OPV modules remains significantly lower than for inorganic devices.
Current research focuses on increasing device efficiency and lifetime. Substantial efficiency gains have been achieved already by improving the absorber material, and research is being done to further optimize the absorbers and develop an organic multijunction architecture. Improved encapsulation and alternative contact materials are being investigated to reduce cell degradation and push cell lifetimes to industry-relevant values.
Learn more about the awardees and the projects involving organic PV below.

• Princeton University (Next Generation Photovoltaics II Projects)
• University of Michigan (Next Generation Photovoltaics II Projects)
• University of Minnesota (Next Generation Photovoltaics II Projects)

Benefits

The benefits promised by OPV solar cells include:

• Low-cost manufacturing: Soluble organic molecules enable roll-to-roll processing techniques and allow for low-cost manufacturing.
• Abundant materials: The wide abundance of building-block materials may reduce supply and price constraints.
• Flexible substrates: The ability to be applied to flexible substrates permits a wide variety of uses.

Production

OPV cells are categorized into two classes:

• Small-molecule OPV cells
• Polymer-based OPV cells.

Small-molecule OPV cells use molecules with broad absorption in the visible and near-infrared portion of the electromagnetic spectrum. Highly conjugated systems are typically used for the electron-donating system such as phthalocyanines, polyacenes, and squarenes. Perylene dyes and fullerenes are often used as the electron-accepting systems. These devices are most commonly generated via vacuum deposition to create bilayer and tandem architectures. Recently, solution-processed small-molecule systems have been developed.
Polymer-based OPV cells use long-chained molecular systems for the electron-donating material (e.g., P3HT, MDMO-PPV), along with derivatized fullerenes as the electron-accepting system (e.g., PC₆₀BM, PC₇₀BM)) Like small-molecule OPV cells, these systems have small exciton diffusion lengths. However, this limitation is circumvented by a high interface surface area within the active device.
Dye-sensitized solar cells are a hybrid organic-inorganic technology that uses small-molecule absorber dyes. These dyes adsorb onto a suitable electron-accepting material, such as titanium dioxide or zinc oxide, along with an electrolyte to regenerate the dye. Unlike their OPV counterparts, dye-sensitized technologies have produced efficiencies beyond 10%; but currently, they require a liquid electrolyte within the solar cell, which limits their durability.